diff --git a/src/wp-admin/css/themes.css b/src/wp-admin/css/themes.css
index 24f50fc51b..7aadbee438 100644
--- a/src/wp-admin/css/themes.css
+++ b/src/wp-admin/css/themes.css
@@ -1644,6 +1644,8 @@ body.full-overlay-active {
 	min-width: 299px;
 	max-width: 599px;
 	width: 18%;
+	width: -webkit-calc( 18% - 1px );
+	width: calc( 18% - 1px );
 	height: 45px;
 	border-top: 1px solid #ddd;
 	background: #eee;