fixes #5443: reset background gradient on disabled dropdown links

docs/assets/css/bootstrap.css

@@ -2859,6 +2859,7 @@ table th[class*="span"],
   text-decoration: none;
   cursor: default;
   background-color: transparent;
+  background-image: none;
 }
 
 .open {

less/dropdowns.less

@@ -116,6 +116,7 @@
 .dropdown-menu .disabled > a:hover {
   text-decoration: none;
   background-color: transparent;
+  background-image: none; // Remove CSS gradient
   cursor: default;
 }