Fix arc theme label backgrounds (#13267)

* Fix arc theme label backgrounds

* Add specific style to yellow labels (background + border color)
* Remove !important from label's background rule to avoid breaking user
  defined labels. Make the rule more specific to override fomantic UI
  default white background

* Remove unnecesary selector

Co-authored-by: zeripath <art27@cantab.net>

web_src/less/themes/theme-arc-green.less

@@ -585,6 +585,12 @@ footer {
   border-color: #8a2121 !important;
 }
 
+.ui.yellow.label,
+.ui.yellow.labels .label {
+  border-color: #664d02 !important;
+  background-color: #936e00 !important;
+}
+
 .ui.menu {
   background: #404552;
   border: 1px solid #353945;
@@ -617,7 +623,7 @@ footer {
 .ui.label.basic {
   color: #dbdbdb;
   border-color: rgb(152, 152, 152);
-  background-color: #2a2e39 !important;
+  background-color: #2a2e39;
 }
 
 .issue.list > .item .title {