Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 37480)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -1645,7 +1645,7 @@
 .theme-install-overlay iframe {
 	height: 100%;
 	width: 100%;
-	z-index: 20;
+	position: relative; /* needed to be on top of the spinner */
 	-webkit-transition: opacity 0.3s;
 	transition: opacity 0.3s;
 }
@@ -1728,7 +1728,6 @@
 
 .theme-install-overlay .wp-full-overlay-main {
 	position: absolute;
-	z-index: 0;
 	background-color: #f1f1f1;
 }
 
@@ -1746,7 +1745,6 @@
 	position: absolute;
 	left: 50%;
 	top: 50%;
-	z-index: -1;
 	margin: -10px 0 0 -10px;
 	-webkit-transform: translateZ(0);
 	transform: translateZ(0);