Add Custom CSS to MCE Editor

gistfile1.php

add_filter('mce_css', 'my_editor_style');
function my_editor_style($url) {

  if ( !empty($url) )
    $url .= ',';

  // Change the path here if using sub-directory
  $url .= trailingslashit( get_stylesheet_directory_uri() ) . 'editor-style.css';

  return $url;
}