[css-color-5] Define light-dark().

Closes #7561.

Author: emilio

css-color-5/Overview.bs

@@ -34,6 +34,8 @@ spec:css-color-4; type:value; text:srgb
 spec:css-color-4; type:dfn; text:modern color syntax
 spec:css-color-4; type:value; text:alpha-value
 spec:css-values-5; type:value; text:color
+spec:css-color-adjust-1; type:value; text:light
+spec:css-color-adjust-1; type:value; text:dark
 </pre>
 <link rel="stylesheet" href="style.css" />
 <style>
@@ -102,8 +104,8 @@ Introduction {#intro}
 	and two colors with visually different lightness,
 	like yellow and blue, can have the same HSL lightness).
 
-	This module adds a new function:
-	''color-mix()'',
+	This module adds the new functions
+	''color-mix()'' and ''light-dark()'',
 	and extends existing ones with [[#relative-colors|relative color syntax]].
 
 	It also extends the ''color()'' function
@@ -121,7 +123,7 @@ Introduction {#intro}
 	Colors in CSS are represented by the <dfn export><<color>></dfn> type:
 
 	<pre class='prod'>
-	&lt;color> = <<absolute-color-base>> | currentcolor | <<system-color>> | <<device-cmyk()>> 
+	&lt;color> = <<absolute-color-base>> | currentcolor | <<system-color>> | <<device-cmyk()>> | <<light-dark()>>
 
 	<dfn>&lt;absolute-color-base></dfn> = <<hex-color>> | <<absolute-color-function>> | <<named-color>> | transparent
 	<dfn>&lt;absolute-color-function></dfn> = <<rgb()>> | <<rgba()>> |
@@ -2468,6 +2470,22 @@ or any other color or monochrome output device which has been characterized.
 			<li>alpha is the same as the input color
 		</ul>
 
+<h2 id="light-dark">
+	Reacting to the used color-scheme: the ''light-dark()'' Function
+</h2>
+
+	System colors have the ability to react to the current used ''color-scheme'' value.
+	The ''light-dark()'' function exposes the same capability to authors.
+
+	<pre class='prod'>
+		<dfn export>light-dark()</dfn> = light-dark( <<color>>, <<color>> )
+	</pre>
+
+	This function computes to the computed value of the first color,
+	if the <a>used color scheme</a> is ''light'' or unknown,
+	or to the computed value of the second color,
+	if the <a>used color scheme</a> is ''dark''.
+
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