Bug 1710324 - Don't specify background color on native scrollbar elements on Windows / Linux. r=Gijs

This allow the automatic scrollbar darkening logic to work without further
tweaks. We don't set appearance: none in any scrollbar on desktop (and we
support scrollbar-{width,color} to customize them anyways).

Differential Revision: https://phabricator.services.mozilla.com/D114696

toolkit/themes/windows/global/xulscrollbars.css

@@ -15,7 +15,6 @@ scrollbar {
   appearance: auto;
   -moz-default-appearance: scrollbar-horizontal;
   cursor: default;
-  background-color: #f0f0f0;
 }
 
 @media all and (-moz-overlay-scrollbars) {
@@ -48,10 +47,6 @@ slider[orient="vertical"] {
 
 /* ::::: thumb (horizontal) ::::: */
 
-thumb {
-  background-color: #cdcdcd;
-}
-
 thumb[orient="vertical"] {
   appearance: auto;
   -moz-default-appearance: scrollbarthumb-vertical;
@@ -71,7 +66,6 @@ scrollcorner {
   -moz-default-appearance: scrollcorner;
   width: 16px;
   cursor: default;
-  background-color: -moz-Dialog;
 }
 
 /* ..... increment .... */