diff --git a/src/wp-admin/css/wp-admin.css b/src/wp-admin/css/wp-admin.css
index 344effdae5..b682e332a1 100644
--- a/src/wp-admin/css/wp-admin.css
+++ b/src/wp-admin/css/wp-admin.css
@@ -1913,7 +1913,7 @@ div.wp-menu-image:before {
 
 .sticky-menu #adminmenuwrap {
 	position: fixed;
-	z-index: 9999; /* Match the z-index of .wp-submenu to ensure flyout menus don't appear underneath main column elements */
+	z-index: 99; /* Match the z-index of .wp-submenu to ensure flyout menus don't appear underneath main column elements */
 }
 
 /* A new arrow */
@@ -7129,7 +7129,7 @@ body.folded .theme-overlay .theme-wrap {
 		left: 0;
 		padding: 70px 20px 20px;
 		border: none;
-		z-index: 999999999;
+		z-index: 500; /* should overlap #wpadminbar, which is 500 on mobile. */
 		position: fixed;
 	}
 
diff --git a/src/wp-includes/js/thickbox/thickbox.css b/src/wp-includes/js/thickbox/thickbox.css
index 2b89963222..26d7106938 100644
--- a/src/wp-includes/js/thickbox/thickbox.css
+++ b/src/wp-includes/js/thickbox/thickbox.css
@@ -23,7 +23,7 @@
 /* ----------------------------------------------------------------------------------------------------------------*/
 #TB_overlay {
 	position: fixed;
-	z-index:100;
+	z-index:510; /* should overlap theme modal, which is 500 on mobile. */
 	top: 0px;
 	left: 0px;
 	height:100%;
@@ -47,7 +47,7 @@
 #TB_window {
 	position: fixed;
 	background: #ffffff;
-	z-index: 102;
+	z-index: 510; /* should overlap theme modal, which is 500 on mobile. */
 	color:#000000;
 	visibility: hidden;
 	text-align:left;