Replace outdated colors with WikimediaUI palette ones

Replacing all colors with current WikimediaUI palette ones.
Also:
- changing two files over to LESS in order to
 - make use of central gradient mixin instead of one with
   unnecessary vendor extensions and
 - concatenating selectors in one occurrence.

Bug: T174237
Change-Id: I5d302f29b079f6abdba5abf3b58d6ba254e1a4ac

resources/css/ext.uls.mixins.less

@@ -14,13 +14,13 @@
 	&.selector-right {
 		&:before {
 			/* @noflip */
-			border-left: 10px solid #c9c9c9;
+			border-left: 10px solid #c8ccd1;
 			/* @noflip */
 			right: -11px;
 		}
 		&:after {
 			/* @noflip */
-			border-left: 10px solid #fcfcfc;
+			border-left: 10px solid #f8f9fa;
 			/* @noflip */
 			right: -10px;
 		}
@@ -28,13 +28,13 @@
 	&.selector-left {
 		&:before {
 			/* @noflip */
-			border-right: 10px solid #c9c9c9;
+			border-right: 10px solid #c8ccd1;
 			/* @noflip */
 			left: -11px;
 		}
 		&:after {
 			/* @noflip */
-			border-right: 10px solid #fcfcfc;
+			border-right: 10px solid #f8f9fa;
 			/* @noflip */
 			left: -10px;
 		}