use CSS variable for select2 border color

Author: tabacitu

src/public/packages/backpack/base/css/blue-bundle.css

@@ -49,7 +49,7 @@ form {
     .select2.select2-container.select2-container--focus,
     .select2.select2-container.select2-container--open {
         background-color: #fff;
-        border: 1px solid #9080f1!important;
+        border: 1px solid var(--primary, #9080f1)!important;
         box-shadow: 0 0 0 2px #e1dcfb;
         color: #495057;
         outline: 0;