ISTWCMS-4083: ensure that not overriding classes, but only adding when putting...

ISTWCMS-4083: ensure that not overriding classes, but only adding when putting in layout builder stuff to identify blocks

uw_fdsu_theme_resp.theme

@@ -244,7 +244,7 @@ function uw_fdsu_theme_resp_preprocess_block(&$variables) {
     // set the class to be used to display the admin label.
     if (!in_array($admin_label, $admin_labels_to_exclude)) {
       $variables['admin_label'] = $admin_label;
-      $variables['attributes']['class'] = 'in-layout-builder';
+      $variables['attributes']['class'][] = 'in-layout-builder';
     }
   }
 }