[css-color-4][editorial] Add system color specified and computed example.

Author: svgeesus

css-color-4/Overview.bs

@@ -5689,6 +5689,15 @@ Resolving values of the ''color()'' function</h3>
 	However, such colors must not be altered by
 	'forced colors mode'.
 
+	<div class="example" id="ex-system-resolve">
+		For example, in this html:
+
+		<pre class="lang-html">&lt;button style="color: ButtonText; background: Buttonface>&lt;/button></pre>
+
+		The specified value of the color property is "ButtonText" 
+		while the computed value could be, for example, rgb(0, 0, 0).
+	</div>
+
 	The specified value of ''transparent'' is "transparent" 
 	while the computed and used value  is [=transparent black=].