Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 29558)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -2677,7 +2677,7 @@
 
 	if ( has_shortcode( $parsed, 'audio' ) || has_shortcode( $parsed, 'video' ) ) {
 		$styles = '';
-		$mce_styles = wp_media_mce_styles();
+		$mce_styles = wpview_media_sandbox_styles();
 		foreach ( $mce_styles as $style ) {
 			$styles .= sprintf( '<link rel="stylesheet" href="%s"/>', $style );
 		}
@@ -2728,7 +2728,7 @@
 
 	ob_start();
 
-	$styles = wp_media_mce_styles();
+	$styles = wpview_media_sandbox_styles();
 	foreach ( $styles as $style ) {
 		printf( '<link rel="stylesheet" href="%s"/>', $style );
 	}
Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 29558)
+++ src/wp-includes/media.php	(working copy)
@@ -3308,11 +3308,20 @@
  *
  * @return array The relevant CSS file URLs.
  */
-function wp_media_mce_styles() {
+function wpview_media_sandbox_styles() {
  	$version = 'ver=' . $GLOBALS['wp_version'];
 	$dashicons = includes_url( "css/dashicons.css?$version" );
  	$mediaelement = includes_url( "js/mediaelement/mediaelementplayer.min.css?$version" );
  	$wpmediaelement = includes_url( "js/mediaelement/wp-mediaelement.css?$version" );
 
-	return array( $dashicons, $mediaelement, $wpmediaelement );
+	/**
+	 * For use by themes that need to override the styling of MediaElement previews in the Visual editor.
+	 * Not intended for adding editor-style.css. Ideally these styles will be applied by using
+	 * the 'seamless' iframe attribute in the future.
+	 *
+	 * @since 4.0
+	 *
+	 * @param array The URLs to the stylesheets that will be loaded in the sandbox iframe.
+	 */	 	 	 	 	
+	return apply_filters( 'wpview_media_sandbox_styles', array( $dashicons, $mediaelement, $wpmediaelement ) );
 }