Use the same colors for admin top-links

Doing so will help to immediately identify which administration you're
managing when using multitenant applications.

We might revert this change if we detect it causes usability issues in
the admin section.

app/assets/stylesheets/admin.scss

@@ -61,8 +61,6 @@ $table-header:   #ecf1f6;
   }
 
   .top-links {
-    background: #000;
-    color: $white;
 
     a {
       line-height: rem-calc($line-height * 1.5);