Replace outdated colors with WikimediaUI palette ones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-monobook.css

@@ -1,3 +1,3 @@
-#pt-uls { /* stylelint-disable selector-no-id */
+#pt-uls { /* stylelint-disable-line selector-no-id */
 	text-transform: none;
 }