Style WordPress Editor

To style WordPress visual editor using CSS add this code snippet to your functions.php file.

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;
}