Colorbox Module won't arrange thumbnails


I’ve a very annoying problem with Colorbox (newest release) and Drupal 8. I tried so solve it since 4 days, read everything I could found and nothing worked. 🙁

As you could see in this link, the thumbnails are arranged vertical. I try to get the thumbnails horizontal. I inspect the site with firebug and tried to modify several css files. It have had no effects.

Could everybody tell me, what file(s) I have to change and what code I have to put in it?

Many, many thanks!

Drupal version: