Index: wp-admin/customize.php
===================================================================
--- wp-admin/customize.php	(revision 28867)
+++ wp-admin/customize.php	(working copy)
@@ -108,8 +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 class="back customize-controls-close" href="<?php echo esc_url( $return ? $return : admin_url( 'themes.php' ) ); ?>"></a> 
 			</a>
 		</div>
 
Index: wp-admin/css/customize-controls.css
===================================================================
--- wp-admin/css/customize-controls.css	(revision 28867)
+++ wp-admin/css/customize-controls.css	(working copy)
@@ -212,10 +212,30 @@
 #customize-header-actions a.back {
 	position: relative;
 	left: 0;
+	visibility: hidden;
 	-webkit-transition: left ease-in-out .18s;
 	transition: left ease-in-out .18s;
 }
 
+.customize-controls-close:before {
+	font: normal 30px/1.5 dashicons;
+	color: #000;
+	width:30px;
+	padding: 0 7.5px 0 7.5px;  
+ 	content: "\f335";  
+ 	position: fixed; 
+ 	visibility: visible;
+	top: 0px;
+	left: 0;
+	border-right: 1px solid #ddd;
+}
+
+.customize-controls-close:hover::before {  
+	background-color: #0074a2;  
+	color: #fff;  
+ 	outline: none; 
+}  
+
 .in-sub-panel #customize-header-actions a.back {
 	left: -120px;
 }