Index: src/wp-admin/customize.php
===================================================================
--- src/wp-admin/customize.php	(revision 28867)
+++ src/wp-admin/customize.php	(working copy)
@@ -108,9 +108,7 @@
 				submit_button( $save_text, 'primary save', 'save', false );
 			?>
 			<span class="spinner"></span>
-			<a class="back button" href="<?php echo esc_url( $return ? $return : admin_url( 'themes.php' ) ); ?>">
-				<?php _e( 'Cancel' ); ?>
-			</a>
+			<a class="cross" href="<?php echo esc_url( $return ? $return : admin_url( 'themes.php' ) ); ?>"></a>
 		</div>
 
 		<?php
Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 28867)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -209,20 +209,53 @@
 	height: 22px;
 }
 
-#customize-header-actions a.back {
+#customize-header-actions a.cross {
 	position: relative;
 	left: 0;
 	-webkit-transition: left ease-in-out .18s;
 	transition: left ease-in-out .18s;
 }
 
-.in-sub-panel #customize-header-actions a.back {
-	left: -120px;
+a.cross:before {
+	font: normal 29px/1 dashicons;
+	content: "\f335";
+	position: relative;
+	top: 9px;
+	left: 9px;
 }
 
+a.cross {
+	color: #444;
+	display: block;
+	position: fixed;
+	top: 0;
+	z-index: 99;
+	left: -48px;
+	width: 45px;
+	height: 45px;
+	padding-right: 2px;
+	background: #eee;
+	border-right: 1px solid #ddd;
+	cursor: pointer;
+	-webkit-transition: left ease-in-out .18s, color ease-in .1s;
+	transition: left ease-in-out .18s, color ease-in .1s;
+}
+
+a.cross:hover,
+a.cross:focus {
+	background-color: #0074a2;
+	color: #fff;
+	outline: none;
+}
+
+.in-sub-panel #customize-header-actions a.cross {
+	left: -129px;
+}
+
 .wp-full-overlay-sidebar .wp-full-overlay-header {
 	-webkit-transition: padding ease-in-out .18s;
 	transition: padding ease-in-out .18s;
+	padding: 0 15px 0 0;
 }
 
 .in-sub-panel .wp-full-overlay-sidebar .wp-full-overlay-header {