Update forms.less

All variables should be set in variables.less?

less/forms.less

@@ -437,7 +437,7 @@ input[type="color"] {
 }
 
 // Only right align form labels here when the columns stop stacking
-@media (min-width: 768px) {
+@media (min-width: @screen-tablet) {
   .form-horizontal .control-label {
     text-align: right;
   }