diff --git a/src/wp-admin/css/themes.css b/src/wp-admin/css/themes.css
index 8aed203aec..79124523d6 100644
--- a/src/wp-admin/css/themes.css
+++ b/src/wp-admin/css/themes.css
@@ -1805,7 +1805,7 @@ body.full-overlay-active {
 #customize-container iframe,
 .theme-install-overlay iframe {
 	height: 100%;
-	width: 100%;
+	width: 100vw;
 	z-index: 20;
 	transition: opacity 0.3s;
 }
@@ -1888,6 +1888,7 @@ body.full-overlay-active {
 	position: absolute;
 	z-index: 0;
 	background-color: #f0f0f1;
+	overflow: auto;
 }
 
 .customize-loading #customize-container {