Allow configs to override default CSS theme values in theme() function provided to plugins and configs (tailwindlabs#14359)

Previously, given the following CSS and configuration:

```css
/* app.css */

@theme default {
  --font-size-base: 1.25rem;
  --font-size-base--line-height: 1.5rem;
}
@tailwind utilities;
@config "./config.js";
```

```js
// config.js

export default {
  theme: {
    fontSize: {
      // …
      base: ['1rem', { lineHeight: '1.75rem' }],
    },

    // …
  },
};
```

When a config or a plugin asked for the value of `theme(fontSize.base)`
like so:

```js
// config.js
export default {
  theme: {
    // …

    typography: ({ theme }) => ({
      css: {
        '[class~="lead"]': {
          fontSize: theme('fontSize.base')[0],
          ...theme('fontSize.base')[1],
        },
      }
    }),
  },
};
```

We would instead pull the values from the CSS theme even through they're
marked with `@theme default`. This would cause the incorrect font size
and line height to be used resulting in something like this (in the case
of the typography plugin with custom styles):

  ```css
  .prose [class~="lead"] {
    font-size: 1.25rem;
    line-height: 1.5rem;
  }
  ```

After this change we'll now pull the values from the appropriate place
(the config in this case) and the correct font size and line height will
be used:

  ```css
  .prose [class~="lead"] {
    font-size: 1rem;
    line-height: 1.75rem;
  }
  ```

This will work even when some values are overridden in the CSS theme:

  ```css
  /* app.css */
  @theme default {
    --font-size-base: 1.25rem;
    --font-size-base--line-height: 1.5rem;
  }
  @theme {
    --font-size-base: 2rem;
  }
  @tailwind utilities;
  @config "./config.js";
  ```

which would result in the following CSS:

  ```css
  .prose [class~="lead"] {
    font-size: 2rem;
    line-height: 1.75rem;
  }
  ```

---------

Co-authored-by: Adam Wathan <4323180+adamwathan@users.noreply.github.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>
Co-authored-by: Adam Wathan <adam.wathan@gmail.com>

Author: 3 people

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Ensure there is always CLI feedback on save even when no new classes were found ([#14351](https://github.com/tailwindlabs/tailwindcss/pull/14351))
 - Properly resolve `theme('someKey.DEFAULT')` when all `--some-key-*` keys have a suffix ([#14354](https://github.com/tailwindlabs/tailwindcss/pull/14354))
+- Make sure tuple theme values in JS configs take precedence over `@theme default` values ([#14359](https://github.com/tailwindlabs/tailwindcss/pull/14359))
 
 ## [4.0.0-alpha.23] - 2024-09-05
 

packages/tailwindcss/src/compat/config.test.ts

@@ -1,6 +1,7 @@
 import { describe, test } from 'vitest'
 import { compile } from '..'
 import plugin from '../plugin'
+import { flattenColorPalette } from './flatten-color-palette'
 
 const css = String.raw
 
@@ -230,6 +231,272 @@ test('Variants in CSS overwrite variants from plugins', async ({ expect }) => {
   `)
 })
 
+describe('theme callbacks', () => {
+  test('tuple values from the config overwrite `@theme default` tuple-ish values from the CSS theme', async ({
+    expect,
+  }) => {
+    let input = css`
+      @theme default {
+        --font-size-base: 0rem;
+        --font-size-base--line-height: 1rem;
+        --font-size-md: 0rem;
+        --font-size-md--line-height: 1rem;
+        --font-size-xl: 0rem;
+        --font-size-xl--line-height: 1rem;
+      }
+      @theme {
+        --font-size-base: 100rem;
+        --font-size-md--line-height: 101rem;
+      }
+      @tailwind utilities;
+      @config "./config.js";
+    `
+
+    let compiler = await compile(input, {
+      loadConfig: async () => ({
+        theme: {
+          extend: {
+            fontSize: {
+              base: ['200rem', { lineHeight: '201rem' }],
+              md: ['200rem', { lineHeight: '201rem' }],
+              xl: ['200rem', { lineHeight: '201rem' }],
+            },
+
+            // Direct access
+            lineHeight: ({ theme }) => ({
+              base: theme('fontSize.base[1].lineHeight'),
+              md: theme('fontSize.md[1].lineHeight'),
+              xl: theme('fontSize.xl[1].lineHeight'),
+            }),
+
+            // Tuple access
+            typography: ({ theme }) => ({
+              '[class~=lead-base]': {
+                fontSize: theme('fontSize.base')[0],
+                ...theme('fontSize.base')[1],
+              },
+              '[class~=lead-md]': {
+                fontSize: theme('fontSize.md')[0],
+                ...theme('fontSize.md')[1],
+              },
+              '[class~=lead-xl]': {
+                fontSize: theme('fontSize.xl')[0],
+                ...theme('fontSize.xl')[1],
+              },
+            }),
+          },
+        },
+
+        plugins: [
+          plugin(function ({ addUtilities, theme }) {
+            addUtilities({
+              '.prose': {
+                ...theme('typography'),
+              },
+            })
+          }),
+        ],
+      }),
+    })
+
+    expect(compiler.build(['leading-base', 'leading-md', 'leading-xl', 'prose']))
+      .toMatchInlineSnapshot(`
+      ":root {
+        --font-size-base: 100rem;
+        --font-size-md--line-height: 101rem;
+      }
+      .prose {
+        [class~=lead-base] {
+          font-size: 100rem;
+          line-height: 201rem;
+        }
+        [class~=lead-md] {
+          font-size: 200rem;
+          line-height: 101rem;
+        }
+        [class~=lead-xl] {
+          font-size: 200rem;
+          line-height: 201rem;
+        }
+      }
+      .leading-base {
+        line-height: 201rem;
+      }
+      .leading-md {
+        line-height: 101rem;
+      }
+      .leading-xl {
+        line-height: 201rem;
+      }
+      "
+    `)
+  })
+})
+
+describe('theme overrides order', () => {
+  test('user theme > js config > default theme', async ({ expect }) => {
+    let input = css`
+      @theme default {
+        --color-red: red;
+      }
+      @theme {
+        --color-blue: blue;
+      }
+      @tailwind utilities;
+      @config "./config.js";
+    `
+
+    let compiler = await compile(input, {
+      loadConfig: async () => ({
+        theme: {
+          extend: {
+            colors: {
+              red: 'very-red',
+              blue: 'very-blue',
+            },
+          },
+        },
+      }),
+    })
+
+    expect(compiler.build(['bg-red', 'bg-blue'])).toMatchInlineSnapshot(`
+      ":root {
+        --color-blue: blue;
+      }
+      .bg-blue {
+        background-color: var(--color-blue, blue);
+      }
+      .bg-red {
+        background-color: very-red;
+      }
+      "
+    `)
+  })
+
+  test('user theme > js config > default theme (with nested object)', async ({ expect }) => {
+    let input = css`
+      @theme default {
+        --color-slate-100: #000100;
+        --color-slate-200: #000200;
+        --color-slate-300: #000300;
+      }
+      @theme {
+        --color-slate-400: #100400;
+        --color-slate-500: #100500;
+      }
+      @tailwind utilities;
+      @config "./config.js";
+      @plugin "./plugin.js";
+    `
+
+    let compiler = await compile(input, {
+      loadConfig: async () => ({
+        theme: {
+          extend: {
+            colors: {
+              slate: {
+                200: '#200200',
+                400: '#200400',
+                600: '#200600',
+              },
+            },
+          },
+        },
+      }),
+
+      loadPlugin: async () => {
+        return plugin(({ matchUtilities, theme }) => {
+          matchUtilities(
+            {
+              'hover-bg': (value) => {
+                return {
+                  '&:hover': {
+                    backgroundColor: value,
+                  },
+                }
+              },
+            },
+            { values: flattenColorPalette(theme('colors')) },
+          )
+        })
+      },
+    })
+
+    expect(
+      compiler.build([
+        'bg-slate-100',
+        'bg-slate-200',
+        'bg-slate-300',
+        'bg-slate-400',
+        'bg-slate-500',
+        'bg-slate-600',
+        'hover-bg-slate-100',
+        'hover-bg-slate-200',
+        'hover-bg-slate-300',
+        'hover-bg-slate-400',
+        'hover-bg-slate-500',
+        'hover-bg-slate-600',
+      ]),
+    ).toMatchInlineSnapshot(`
+      ":root {
+        --color-slate-100: #000100;
+        --color-slate-300: #000300;
+        --color-slate-400: #100400;
+        --color-slate-500: #100500;
+      }
+      .bg-slate-100 {
+        background-color: var(--color-slate-100, #000100);
+      }
+      .bg-slate-200 {
+        background-color: #200200;
+      }
+      .bg-slate-300 {
+        background-color: var(--color-slate-300, #000300);
+      }
+      .bg-slate-400 {
+        background-color: var(--color-slate-400, #100400);
+      }
+      .bg-slate-500 {
+        background-color: var(--color-slate-500, #100500);
+      }
+      .bg-slate-600 {
+        background-color: #200600;
+      }
+      .hover-bg-slate-100 {
+        &:hover {
+          background-color: #000100;
+        }
+      }
+      .hover-bg-slate-200 {
+        &:hover {
+          background-color: #200200;
+        }
+      }
+      .hover-bg-slate-300 {
+        &:hover {
+          background-color: #000300;
+        }
+      }
+      .hover-bg-slate-400 {
+        &:hover {
+          background-color: #100400;
+        }
+      }
+      .hover-bg-slate-500 {
+        &:hover {
+          background-color: #100500;
+        }
+      }
+      .hover-bg-slate-600 {
+        &:hover {
+          background-color: #200600;
+        }
+      }
+      "
+    `)
+  })
+})
+
 describe('default font family compatibility', () => {
   test('overriding `fontFamily.sans` sets `--default-font-family`', async ({ expect }) => {
     let input = css`

packages/tailwindcss/src/compat/config/deep-merge.ts

@@ -10,7 +10,8 @@ export function isPlainObject<T>(value: T): value is T & Record<keyof T, unknown
 export function deepMerge<T extends object>(
   target: T,
   sources: (Partial<T> | null | undefined)[],
-  customizer: (a: any, b: any) => any,
+  customizer: (a: any, b: any, keypath: (keyof T)[]) => any,
+  parentPath: (keyof T)[] = [],
 ) {
   type Key = keyof T
   type Value = T[Key]
@@ -21,14 +22,20 @@ export function deepMerge<T extends object>(
     }
 
     for (let k of Reflect.ownKeys(source) as Key[]) {
-      let merged = customizer(target[k], source[k])
+      let currentParentPath = [...parentPath, k]
+      let merged = customizer(target[k], source[k], currentParentPath)
 
       if (merged !== undefined) {
         target[k] = merged
       } else if (!isPlainObject(target[k]) || !isPlainObject(source[k])) {
         target[k] = source[k] as Value
       } else {
-        target[k] = deepMerge({}, [target[k], source[k]], customizer) as Value
+        target[k] = deepMerge(
+          {},
+          [target[k], source[k]],
+          customizer,
+          currentParentPath as any,
+        ) as Value
       }
     }
   }

packages/tailwindcss/src/compat/flatten-color-palette.ts

@@ -0,0 +1,19 @@
+type Colors = {
+  [key: string | number]: string | Colors
+}
+
+export function flattenColorPalette(colors: Colors) {
+  let result: Record<string, string> = {}
+
+  for (let [root, children] of Object.entries(colors ?? {})) {
+    if (typeof children === 'object' && children !== null) {
+      for (let [parent, value] of Object.entries(flattenColorPalette(children))) {
+        result[`${root}${parent === 'DEFAULT' ? '' : `-${parent}`}`] = value
+      }
+    } else {
+      result[root] = children
+    }
+  }
+
+  return result
+}