Index: wp-includes/script-loader.php
===================================================================
--- wp-includes/script-loader.php	(revision 28157)
+++ wp-includes/script-loader.php	(working copy)
@@ -635,7 +635,7 @@
 	$styles->add( 'wp-jquery-ui-dialog', "/wp-includes/css/jquery-ui-dialog$suffix.css", array( 'dashicons' ) );
 	$styles->add( 'mediaelement',        "/wp-includes/js/mediaelement/mediaelementplayer.min.css", array(), '2.13.0' );
 	$styles->add( 'wp-mediaelement',     "/wp-includes/js/mediaelement/wp-mediaelement.css", array( 'mediaelement' ) );
-	$styles->add( 'thickbox',            '/wp-includes/js/thickbox/thickbox.css', array( 'dashicons' ), '20131201' );
+	$styles->add( 'thickbox',            '/wp-includes/js/thickbox/thickbox.css', array( 'dashicons' ) );
 
 	// Deprecated CSS
 	$styles->add( 'media',      "/wp-admin/css/deprecated-media$suffix.css" );