[css-color-adjust-1] Add note highlighting that color schemes don't guarantee particular colors. w3c#3983

Author: fantasai

css-color-adjust-1/Overview.bs

@@ -60,6 +60,16 @@ Preferred Color Schemes {#preferred}
 	defines controls to enable [=color scheme=] negotiation
 	for “Light” and “Dark” [=color schemes=] (and no preference).
 
+	Note: <strong>The “Light” and “Dark” color schemes
+	don't represent an exact color palette (such as black-and-white),
+	but a range of possible palettes.
+	To guarantee specific colors, authors must specify those colors themselves.</strong>
+	Note also that, consequently,
+	pairing default or <<system-color>> colors with author-specified colors
+	cannot guarantee any particular contrast level;
+	it might be necessary to set both foreground and background colors together
+	to ensure legibility [[WCAG21]].
+
 	To enable pages to adapt to the user's preferred [=color scheme=],
 	user agents will match the '@media/prefers-color-scheme' media query
 	to the user's preferred [=color schemes=].