rustdoc light theme: Fix CSS for selected buttons

The background was dark before, which made the text impossible to read.
Now the background is white, which is how selected `div`s are rendered.

As a result, the search results tabs now look identical to how they
used to look (before rust-lang#79896).

src/librustdoc/html/static/themes/light.css

@@ -349,8 +349,8 @@ pre.ignore:hover, .information:hover + pre.ignore {
 }
 
 #titles > button:hover, #titles > button.selected {
+	background-color: #ffffff;
 	border-top-color: #0089ff;
-	background-color: #353535;
 }
 
 #titles > button > div.count {