styles: Replace 'mediawiki.ui/variables' call with skin variables

- Replacing 'mediawiki.ui/variables.less' `@import`
  with new Codex design tokens as external library. We're using the
  legacy values as UniversalLanguageSelector has been a desktop first
  extension and those values are also used by Vector skin.
  The values replaced are anyways only touching colors, borders or
  transitions which are the same in normal and legacy.
  These will be replaced by skin-aware 'mediawiki.skin.variables.less'
  standard with dependency of UniversalLanguageSelector upped
  to MW 1.42.0.
- Replacing several static values with new Codex design token featuring
  skin variables, in categories `background-color`, `color`,
  `border-*`, `box-shadow`, `opacity` and `transition`.

Note, that this change should result in same rendering of the
extension, only properties that already have an equal token are
replaced.

Bug: T332541
Co-Authored-by: Volker E. <volker.e@wikimedia.org>
Change-Id: Ic563fa84bf8e76fd62f14487924d7d6fd5c1a065

resources/css/ext.uls.less

@@ -1,21 +1,23 @@
+// TODO: Replace with @import 'mediawiki.skin.variables.less'; with MW core LTS v1.42 dependency.
+@import '../../lib/codex-design-tokens/theme-wikimedia-ui-legacy.less';
 @import 'ext.uls.mixins.less';
 
 /* Overrides to follow MediaWiki style */
 .uls-menu {
-	border-radius: 2px;
+	border-radius: @border-radius-base;
 	/* Ensure base font size is same in different skins */
 	font-size: medium;
 }
 
 .uls-search,
 .uls-language-settings-close-block {
-	border-top-right-radius: 2px;
-	border-top-left-radius: 2px;
+	border-top-right-radius: @border-radius-base;
+	border-top-left-radius: @border-radius-base;
 }
 
 .uls-language-list {
-	border-bottom-right-radius: 2px;
-	border-bottom-left-radius: 2px;
+	border-bottom-right-radius: @border-radius-base;
+	border-bottom-left-radius: @border-radius-base;
 }
 
 .uls-menu.callout {
@@ -43,8 +45,8 @@
 	display: block;
 	position: absolute;
 	left: 0;
-	border-right: 1px solid #c8ccd1;
-	opacity: 0.8;
+	border-right: @border-width-base @border-style-base @border-color-subtle;
+	opacity: @opacity-icon-base;
 }
 
 .uls-icon-back:hover {
@@ -54,13 +56,13 @@
 
 .uls-menu .uls-no-results-view {
 	.uls-no-found-more {
-		background-color: #fff;
+		background-color: @background-color-base;
 	}
 
 	h3 {
 		padding: 0 28px;
 		margin: 0;
-		color: #54595d;
+		color: @color-subtle;
 		font-size: 1em;
 		font-weight: normal;
 	}