Ticket #40978: 40978.1.diff

src/wp-admin/css/themes.css

@@ -1644 +1644 @@
         min-width: 299px;
         max-width: 599px;
         width: 18%;
+        width: calc( 18% - 1px );
         height: 45px;
         border-top: 1px solid #ddd;
         background: #eee;