Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 40958)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -370,7 +370,7 @@
 	bottom: 0;
 	background: #f1f1f1;
 	background: rgba( 238, 238, 238, 0.9 );
-	z-index: 10000; /* Over WP Pointers. */
+	z-index: 9989; /* Over WP Pointers. */
 }
 
 .theme-overlay .theme-header {
@@ -481,7 +481,7 @@
 	background: #fff;
 	-webkit-box-shadow: 0 1px 20px 5px rgba(0, 0, 0, 0.1);
 	box-shadow: 0 1px 20px 5px rgba(0, 0, 0, 0.1);
-	z-index: 10000; /* Over WP Pointers. */
+	z-index: 9990; /* Over WP Pointers. */
 	-webkit-box-sizing: border-box;
 	-moz-box-sizing: border-box;
 	box-sizing: border-box;
@@ -801,7 +801,7 @@
 
 @media only screen and (min-width: 1680px) {
 	.theme-overlay .theme-wrap {
-		width: 1450px;
+		width: 1435px; /* Reduced theme overlay width for admin sidebar submenu */
 		margin: 0 auto;
 	}
 }