Index: themes/pub/wporg-themes/css/components/_main.scss
===================================================================
--- sites/trunk/wordpress.org/public_html/wp-content/themes/pub/wporg-themes/css/components/_main.scss	(revision 8011)
+++ sites/trunk/wordpress.org/public_html/wp-content/themes/pub/wporg-themes/css/components/_main.scss	(working copy)
@@ -1001,7 +1001,7 @@
 
 #customize-container iframe,
 .theme-install-overlay iframe {
-	height: 100%;
+	height: calc( 100% - 32px );
 	width: 100%;
 	z-index: 20;
 	transition: opacity 0.3s;