[css-scrollbars] auto is UA defined, may switch between light and dark

See #6438

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -98,7 +98,12 @@ scrollbar-color value set on the HTML body element are not propagated to the vie
 
 <dl dfn-type="value" dfn-for="scrollbar-color">
 <dt><dfn>auto</dfn></dt>
-<dd>default platform rendering for the scrollbar.</dd>
+<dd>The user agent determines the colors of the scrollbar.
+It should follow platform conventions,
+but may adjust the colors depending on context and other properties,
+such as switching between light or dark colors
+to suit the surrounding color scheme.
+</dd>
 <dt><dfn><<color>></dfn></dt>
 <dd>
 apply the first color to the thumb of the scrollbar,