Update select disabled style

jet/static/jet/css/select2/_single.scss

@@ -45,7 +45,7 @@
 
 &.select2-container--disabled {
   .select2-selection--single {
-    background-color: $input-contrast-background-color;
+    background-color: transparentize($input-contrast-background-color, 0.75);
     cursor: default;
 
     .select2-selection__clear {