[css-scrollbar] Editorial: rephasing

Make the prose read well, using full sentences, defined terms and
hyperlinks. No normative change.

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -117,17 +117,17 @@ Value Definitions</h3>
 			to better suit the page.
 		<dt><dfn><<color>></dfn>
 		<dd>
-			apply the first color to the thumb of the scrollbar,
-			and the second color to the track of the scrollbar.
+			The user agent should use the specified colors
+			to adjust the rendering of the element's scollbars, if any,
+			applying the first color to the [=thumb=] of the scrollbar,
+			and the second color to the [=track=] of the scrollbar.
 	</dl>
 
-	Details:
-
-	Track refers to the background of the scrollbar,
+	The <dfn>track</dfn> refers to the background of the scrollbar,
 	which is generally fixed regardless of the scrolling position.
 
-	Thumb refers to the moving part of the scrollbar,
-	which usually floats on top of the track.
+	The <dfn>thumb</dfn> refers to the moving part of the scrollbar,
+	which usually floats on top of the [=track=].
 
 	If this property computes to a value other than ''scrollbar-color/auto'',
 	implementations may render a simpler scrollbar than the default platform UI rendering,