Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 40891)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -1644,6 +1644,7 @@
 	min-width: 299px;
 	max-width: 599px;
 	width: 18%;
+	width: calc( 18% - 1px );
 	height: 45px;
 	border-top: 1px solid #ddd;
 	background: #eee;