WordPress native “playlist” shortcode. Next and Prev there are but with no icons. How to fix?

If I use on my WordPress 4.0 the beautiful playlist shortcode I don’t see the famous “Next” and “Prev” icons, but only this code in my Html code:

<div class="wp-playlist-next"></div>

and

<div class="wp-playlist-prev"></div>

How to display icons to next and prev commands?

in stylesheet:

.wp-playlist-prev{
   width:20px;
   height:20px;
   background-image: url("./theme_images_folder/something_icon.png");
}

same for the next button.