Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 40430)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -1188,21 +1188,18 @@
 }
 .customize-section-description-container + #customize-control-custom_css:last-child {
 	margin-left: -12px;
-	width: 299px;
+	width: -webkit-calc( 100% + 24px );
+	width: calc( 100% + 24px );
 	margin-bottom: -12px;
 }
 
 @media screen and ( max-width: 640px ) {
 	.customize-section-description-container + #customize-control-custom_css:last-child {
-		margin-left: 0;
 		margin-right: 0;
-		width: 100%;
 	}
 	.customize-section-description-container + #customize-control-custom_css:last-child textarea {
 		height: -webkit-calc( 100vh - 140px );
 		height: calc( 100vh - 140px );
-		width: 100%;
-		border: solid 1px #ddd;
 	}
 }
 
Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 40430)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -1332,7 +1332,9 @@
 	-moz-box-sizing: border-box;
 	box-sizing: border-box;
 	position: fixed;
-	width: 300px;
+	min-width: 300px;
+	max-width: 600px;
+	width: 18%;
 	height: 100%;
 	top: 0;
 	bottom: 0;
@@ -1361,6 +1363,26 @@
 	margin-left: -300px;
 }
 
+@media screen and (min-width: 1667px) {
+	.wp-full-overlay.expanded {
+		margin-left: 18%;
+	}
+
+	.wp-full-overlay.collapsed .wp-full-overlay-sidebar {
+		margin-left: -18%;
+	}
+}
+
+@media screen and (min-width: 3333px) {
+	.wp-full-overlay.expanded {
+		margin-left: 600px;
+	}
+
+	.wp-full-overlay.collapsed .wp-full-overlay-sidebar {
+		margin-left: -600px;
+	}
+}
+
 .wp-full-overlay-sidebar:after {
 	content: "";
 	display: block;
@@ -1616,7 +1638,9 @@
 	position: fixed;
 	bottom: 0;
 	left: 0;
-	width: 299px;
+	min-width: 299px;
+	max-width: 599px;
+	width: 18%;
 	height: 45px;
 	border-top: 1px solid #ddd;
 	background: #eee;