DEV: Remove SCSS color vars fallbacks (#112)

assets/stylesheets/adplugin.scss

@@ -180,7 +180,7 @@
   text-transform: uppercase;
   line-height: 1;
   letter-spacing: 1px;
-  color: var(--quaternary, $quaternary) !important;
+  color: var(--quaternary) !important;
 }
 
 .adbutler-ad {
@@ -229,7 +229,7 @@
     p.help {
       margin: 0;
       margin-top: 5px;
-      color: var(--primary-medium, $primary-medium);
+      color: var(--primary-medium);
       font-size: $font-down-1;
       clear: both;
     }