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‎docs/theming.md

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# MDL V2 Theming Guide
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## Overview
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MDL includes a theming system designed to make it easy to change your application's colors. It provides multiple options
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for implementing themes, allowing for maximum flexibility. At the moment, MDL supports theming with Sass and with CSS
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Custom Property, with plans for CDN support as well, once that service is available.
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## Colors
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MDL theming, like Material Design theming, uses two main colors: **primary** and **accent**. The primary color is used
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throughout most of the application and components, as the main color for your application. The accent color is used
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for floating action buttons and other interactive elements, serving as visual contrast to the primary.
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In addition to the primary and accent colors, MDL also defines a _background_ color, which is used as a background in
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components, and usually as the page background as well.
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Finally, MDL has a number of text colors, which are used for rendering text and other shapes on top of the primary,
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accent and background colors. These are specified as either dark or light, in order to provide sufficient contrast to
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what's behind them, and have
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[different levels of opacity depending on usage](https://material.google.com/style/color.html#color-color-schemes):
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- Primary, used for most text.
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- Secondary, used for text which is lower in the visual hierarchy.
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- Hint, used for text hints (such as those in text fields and labels).
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- Disabled, used for text in disabled components and content.
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- Icon, used for icons.
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## Building a themed application
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Let's start with a simple application, which displays several cards for different categories. We ultimately want each
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card to have a color scheme that matches its category, but we'll start with the default theming provided by MDL.
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You can [take a look at the end result here](https://plnkr.co/edit/qxt7qpPsXgkt9UbMnE36?p=preview), but let's start from
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scratch.
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> Note: We won't cover the basics of starting an MDL v2 project in this guide, so please take a look at the
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[getting started guide](./getting-started.md) if you need more information.
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### Step 1: No theming
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Here's the markup:
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```html
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<!DOCTYPE html>
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<html class="mdl-typography">
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<head>
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<meta charset="utf-8">
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<meta name="viewport" content="width=device-width,initial-scale=1">
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<title>Elements</title>
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<link rel="stylesheet" href="/node_modules/material-design-lite/dist/material-design-lite.css">
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<style>
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.cards {
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display: flex;
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flex-wrap: wrap;
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}
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.element-card {
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width: 16em;
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margin: 16px;
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}
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.element-card > .mdl-card__media {
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height: 9em;
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}
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</style>
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</head>
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<body>
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<h1>Choose your element</h1>
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<div class="cards">
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<div class="mdl-card element-card earth">
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<section class="mdl-card__media">
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<h1 class="mdl-card__title mdl-card__title--large">Earth</h1>
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<h2 class="mdl-card__subtitle">A solid decision.</h2>
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</section>
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<section class="mdl-card__actions">
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<button class="mdl-button mdl-button--primary mdl-button--compact mdl-card__action">Pick this element</button>
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</section>
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</div>
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<div class="mdl-card element-card wind">
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<section class="mdl-card__media">
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<h1 class="mdl-card__title mdl-card__title--large">Wind</h1>
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<h2 class="mdl-card__subtitle">Stormy weather ahead.</h2>
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</section>
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<section class="mdl-card__actions">
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<button class="mdl-button mdl-button--primary mdl-button--compact mdl-card__action">Pick this element</button>
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</section>
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</div>
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<div class="mdl-card element-card fire">
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<section class="mdl-card__media">
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<h1 class="mdl-card__title mdl-card__title--large">Fire</h1>
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<h2 class="mdl-card__subtitle">Hot-headed much?</h2>
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</section>
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<section class="mdl-card__actions">
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<button class="mdl-button mdl-button--primary mdl-button--compact mdl-card__action">Pick this element</button>
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</section>
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</div>
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<div class="mdl-card element-card water">
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<section class="mdl-card__media">
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<h1 class="mdl-card__title mdl-card__title--large">Water</h1>
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<h2 class="mdl-card__subtitle">Go with the flow.</h2>
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</section>
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<section class="mdl-card__actions">
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<button class="mdl-button mdl-button--primary mdl-button--compact mdl-card__action">Pick this element</button>
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</section>
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</div>
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</div>
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</body>
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</html>
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```
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You'll see that we have a number of pretty empty looking cards, with black text on a white background. The only hint of
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color comes from the buttons, which we've made use the primary color by adding the `mdl-button--primary` class.
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### Step 2: Use the MDL colors in your own markup
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Not everything has a `--primary` option, though, particularly where it comes to your own markup.
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Let's make things look a bit more interesting, by using the primary color as a background to the cards' media area.
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One way of doing this would be to write your own custom CSS rules, and set the background to the same color that's being
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used as the primary:
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```css
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/* Bad approach */
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.element-card > .mdl-card__media {
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background-color: #3f51b5;
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}
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```
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However, that would not take advantage of MDL's theming and would thus be brittle; changes to theming would need to be
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copied to your CSS rules, adding a maintenance cost.
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MDL provides a number of CSS classes as part of the `mdl-theme` module to help you tackle this problem in a more
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maintainable way. Here are the classes that deal with primary, accent and background colors:
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| Class | Description |
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| ----------------------- | ----------------------------------------------------------- |
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| `mdl-theme--primary` | Sets the text color to the theme primary color. |
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| `mdl-theme--accent` | Sets the text color to the theme accent color. |
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| `mdl-theme--background` | Sets the background color to the theme background color. |
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| `mdl-theme--primary-bg` | Sets the background color to the theme primary color. |
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| `mdl-theme--accent-bg` | Sets the background color to the theme accent color. |
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From here, we can see that we want to apply `mdl-theme--primary-bg` to the cards' media areas:
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```html
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<div class="mdl-card element-card">
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<section class="mdl-card__media mdl-theme--primary-bg">
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<h1 class="mdl-card__title mdl-card__title--large">Earth</h1>
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<h2 class="mdl-card__subtitle">A solid decision.</h2>
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</section>
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<section class="mdl-card__actions">
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<button class="mdl-button mdl-button--primary mdl-button--compact mdl-card__action">Pick this element</button>
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</section>
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</div>
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```
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All the cards now use the default primary color (Indigo 500 from the Material palette) as the background for the media
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area, the same color that's being used for the text in the buttons.
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However, you'll notice that the text in the media area is still black, which provides very little contrast to the
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default primary color. Not all primary colors are dark, though, so you can't just switch the text color to white and
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call it a day. Ideally, we want a solution which is as maintainable as the `mdl-theme--primary-bg` class, and which
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takes into account the primary color, in order to determine whether to overlay white or black text on top.
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`mdl-theme` provides utility classes for that purpose as well. Namely, for overlaying text on a primary color
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background, there are:
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| Class | Description |
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| -------------------------------------- | ----------------------------------------------------------------------------------------- |
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| `mdl-theme--text-primary-on-primary` | Set text to suitable color for primary text on top of a theme primary color background. |
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| `mdl-theme--text-secondary-on-primary` | Set text to suitable color for secondary text on top of a theme primary color background. |
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| `mdl-theme--text-hint-on-primary` | Set text to suitable color for hint text on top of a theme primary color background. |
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| `mdl-theme--text-disabled-on-primary` | Set text to suitable color for disabled text on top of a theme primary color background. |
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| `mdl-theme--text-icon-on-primary` | Set text to suitable color for icons on top of a theme primary color background. |
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> Note: `primary`, `secondary`, `hint`, `disabled` and `icon` refer to the text's function. The fact that we use the
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word _primary_ in the different contexts of _primary color_ and _primary function text_ can be confusing at first.
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From here, we can see the right choice is `mdl-theme--text-primary-on-primary`. We could think of applying it to the
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media area, but that won't work because of scoping issues. If we apply it directly to the title and subtitle, though:
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```html
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<div class="mdl-card element-card">
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<section class="mdl-card__media mdl-theme--primary-bg">
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<h1 class="mdl-card__title mdl-card__title--large mdl-theme--text-primary-on-primary">Earth</h1>
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<h2 class="mdl-card__subtitle mdl-theme--text-primary-on-primary">A solid decision.</h2>
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</section>
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<section class="mdl-card__actions">
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<button class="mdl-button mdl-button--primary mdl-button--compact mdl-card__action">Pick this element</button>
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</section>
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</div>
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```
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The text is now white, which provides much better contrast. If we were to change the primary color to a light color,
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however, the text would be dark again, for the same reason. So how _do_ we change the primary color?
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### Step 3: Changing the theme with Sass
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The application-wide theme colors that are used as the default across your entire application can be set in Sass.
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This is as easy as defining three variables (`$mdl-theme-primary`, `$mdl-theme-accent` and `$mdl-theme-background`) in
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your Sass file, before importing any MDL modules.
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```scss
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// My main Sass file.
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$mdl-theme-primary: #9c27b0;
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$mdl-theme-accent: #76ff03;
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$mdl-theme-background: #fff;
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@import "material-design-lite";
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```
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These definitions will override the defaults included in the `mdl-theme` module, which every themeable component depends
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on. As for the text colors, these will all be automatically calculated from the primary, accent and background you
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provide, as part of the Sass definitions in `mdl-theme`. Pretty simple!
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> Note: theme colors don't have to be part of the Material palette; you can use any valid color. You may want to read
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the [color section](https://material.google.com/style/color.html) in the Material Design spec to inform your pick of an
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alternative palette.
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### Step 4: Changing the theme with CSS Custom Properties
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Changing the theme colors with Sass affects the whole application, which is great if you want consistency across the
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board. What we want here is slightly different, though: we want each card to have its own internally consistent theme.
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So how do we keep all the current theme color "plumbing" for maintainability, while having different themes in different
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places? CSS Custom properties to the rescue!
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The generated MDL CSS uses CSS Custom Properties with hardcoded fallbacks, which are set to the colors provided in Sass.
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This means that you can define your default theme in Sass (like we did above), but override it in CSS, dependent on
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context or user preference.
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Let's take a closer look at how MDL does things. Here's an excerpt of a compiled MDL CSS rule:
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```css
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.mdl-button--primary.mdl-button--raised {
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background-color: #3f51b5;
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background-color: var(--mdl-theme-primary, #3f51b5);
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}
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```
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Here, you can see that MDL sets a fallback for the color, for browsers that don't support CSS Custom Properties. If
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they do, however, that declaration gets overriden by a `var()` lookup, using the same fallback as the default value
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(in case the custom property definition isn't found).
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As such, you can easily override the colors that get used in MDL components by simply redefining the custom property at
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some level. So if we want to apply it to our cards, we can take advantage of the element classes we had set up:
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```css
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.element-card.earth {
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--mdl-theme-primary: #795548;
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}
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.element-card.wind {
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--mdl-theme-primary: #9e9e9e;
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}
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.element-card.fire {
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--mdl-theme-primary: #f44336;
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}
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.element-card.water {
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--mdl-theme-primary: #00bcd4;
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}
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```
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It works! You can see that the colors get applied to both the backgrounds and the buttons. If the cards had any other
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components, they'd use the correct colors as well.
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The custom properties used by MDL follow a similar naming convention to the Sass variables and CSS classes:
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| Custom property | Description |
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| ------------------------ | --------------------------- |
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| `--mdl-theme-primary` | The theme primary color. |
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| `--mdl-theme-accent` | The theme accent color. |
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| `--mdl-theme-background` | The theme background color. |
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However, if you look closely at the page, we're not quite done yet. The text colors are incorrect: the wind and water
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cards should have dark text, rather than white. So what's happening?
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The problem is that we only set the `--mdl-theme-primary` custom property. Whereas setting `$mdl-theme-primary` in Sass
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allows for calculating all the related text colors, it's currently not possible to perform those complex contrast
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calculations in CSS. This means you'll also have to set all the related text colors:
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| Custom property | Description |
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| ------------------------------------------ | ---------------------------------------------------------- |
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| `--mdl-theme-text-primary-on-primary` | Primary text on top of a theme primary color background. |
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| `--mdl-theme-text-secondary-on-primary` | Secondary text on top of a theme primary color background. |
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| `--mdl-theme-text-hint-on-primary` | Hint text on top of a theme primary color background. |
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| `--mdl-theme-text-disabled-on-primary` | Disabled text on top of a theme primary color background. |
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| `--mdl-theme-text-icon-on-primary` | Icons on top of a theme primary color background. |
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The same pattern is followed for text colors on _accent_ and _background_:
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| Custom property | Description |
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| ------------------------------------------ | ---------------------------------------------------------- |
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| `--mdl-theme-text-primary-on-accent` | Primary text on top of a theme accent color background. |
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| `--mdl-theme-text-secondary-on-accent` | Secondary text on top of a theme accent color background. |
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| `--mdl-theme-text-hint-on-accent` | Hint text on top of a theme accent color background. |
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| `--mdl-theme-text-disabled-on-accent` | Disabled text on top of a theme accent color background. |
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| `--mdl-theme-text-icon-on-accent` | Icons on top of a theme accent color background. |
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| Custom property | Description |
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| ------------------------------------------ | ---------------------------------------------------------- |
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| `--mdl-theme-text-primary-on-background` | Primary text on top of the theme background color. |
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| `--mdl-theme-text-secondary-on-background` | Secondary text on top of the theme background color. |
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| `--mdl-theme-text-hint-on-background` | Hint text on top of the theme background color. |
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| `--mdl-theme-text-disabled-on-background` | Disabled text on top of the theme background color. |
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| `--mdl-theme-text-icon-on-background` | Icons on top of the theme background color. |
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In addition, we also define custom properties for known dark and light backgrounds:
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| Custom property | Description |
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| ------------------------------------------ | ---------------------------------------------------------- |
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| `--mdl-theme-text-primary-on-light` | Primary text on top of a light-colored background. |
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| `--mdl-theme-text-secondary-on-light` | Secondary text on top of a light-colored background. |
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| `--mdl-theme-text-hint-on-light` | Hint text on top of a light-colored background. |
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| `--mdl-theme-text-disabled-on-light` | Disabled text on top of a light-colored background. |
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| `--mdl-theme-text-icon-on-light` | Icons on top of a light-colored background. |
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| Custom property | Description |
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| ------------------------------------------ | ---------------------------------------------------------- |
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| `--mdl-theme-text-primary-on-dark` | Primary text on top of a dark-colored background. |
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| `--mdl-theme-text-secondary-on-dark` | Secondary text on top of a dark-colored background. |
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| `--mdl-theme-text-hint-on-dark` | Hint text on top of a dark-colored background. |
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| `--mdl-theme-text-disabled-on-dark` | Disabled text on top of a dark-colored background. |
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| `--mdl-theme-text-icon-on-dark` | Icons on top of a dark-colored background. |
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Ideally, we should set all of the text colors on primary, since we never know which one an MDL component might use.
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Since we're just using buttons, though, let's keep it simple for now:
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```css
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.element-card.earth {
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--mdl-theme-primary: #795548;
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--mdl-theme-text-primary-on-primary: var(--mdl-theme-text-primary-on-dark);
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}
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.element-card.wind {
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--mdl-theme-primary: #9e9e9e;
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--mdl-theme-text-primary-on-primary: var(--mdl-theme-text-primary-on-light);
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}
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.element-card.fire {
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--mdl-theme-primary: #f44336;
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--mdl-theme-text-primary-on-primary: var(--mdl-theme-text-primary-on-dark);
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}
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.element-card.water {
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--mdl-theme-primary: #00bcd4;
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--mdl-theme-text-primary-on-primary: var(--mdl-theme-text-primary-on-light);
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}
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```
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> Note: in the future we plan to provide a Javascript utility method for changing all derived colors and making this
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use-case easier.
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## Dark Themes
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Beyond what we've covered in this document so far, there's also the concept of a _dark theme_. All MDL components have
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been designed to work with both light themes (that assume a light-colored background) and dark themes (with dark-colored
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backgrounds), but the default is always light.
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> Note: When using a dark theme, you probably want to choose a dark color as the background for your page, and adjust
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the MDL `background` color to match.
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In order to apply a dark theme to a single element, you can use its `--dark` class. For example, for a button:
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```html
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<button class="mdl-button mdl-button--raised mdl-button--dark">
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Raised dark button
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</button>
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```
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Alternatively, you can set your entire page (or a portion of it) to a dark theme by using the `mdl-theme--dark` class:
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```html
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<section class="mdl-theme--dark">
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<button class="mdl-button mdl-button--raised">
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Still dark
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</button>
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<button class="mdl-button">
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Me too!
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</button>
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</section>
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```
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> Note: there's currently no way to set a light portion inside of a dark one, so if you want to achieve that effect
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you'll need to selectively apply dark classes to everything except the light bits.

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