Replace outdated colors with WikimediaUI palette ones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.interlanguage.less

@@ -1,6 +1,5 @@
-@import 'mediawiki.mixins';
-
 /* stylelint-disable selector-no-id */
+@import 'mediawiki.mixins';
 
 #p-lang .uls-settings-trigger {
 	background: transparent no-repeat right top;