Hello, @Oliver Theiß,
Alright, For the single product page I have added CSS code in order to solve your issue. See the image of the code: https://postimg.cc/GTwk0DRM and now check the result images of the single product page of your site: https://postimg.cc/qhftgdFq and https://postimg.cc/DmnrcqCZ Now please check back to your site after clearing the browser cache.
I hope it’s Done.
Regards 8Themes Team.