[css-scrollbar] Add scrollbar-color: dark/light

Author: upsuper

css-scrollbars-1/Overview.bs

@@ -67,7 +67,7 @@ Values</h3>
 
 <pre class="propdef">
 Name: scrollbar-color
-Value: auto | <<color>>{2}
+Value: auto | dark | light | <<color>>{2}
 Initial: auto
 Inherited: yes
 Computed value: two computed colors or the keyword auto
@@ -86,6 +86,18 @@ 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>
+<dt><dfn>dark</dfn></dt>
+<dd>
+show a dark scrollbar,
+which can be either a dark variant of scrollbar provided by the platform,
+or a custom scrollbar with dark colors.
+</dd>
+<dt><dfn>light</dfn></dt>
+<dd>
+show a light scrollbar,
+which can be either a light variant of scrollbar provided by the platform,
+or a custom scrollbar with light colors.
+</dd>
 <dt><dfn><<color>></dfn></dt>
 <dd>
 apply the first color to the thumb of the scrollbar,