Index: wp-admin/css/customize-controls.css
===================================================================
--- wp-admin/css/customize-controls.css	(revision 36494)
+++ wp-admin/css/customize-controls.css	(working copy)
@@ -806,6 +806,9 @@
 	position: relative;
 	text-align: center;
 	cursor: default;
+        border: 1px dashed #ccc;
+        box-sizing: border-box;
+        height: 40px !important;
 }
 
 .customize-control-media .inner,