Hello, @Oliver Theiß,
I have just checked your site and the colors are working perfectly fine for me, please see this screenshot: https://postimg.cc/GTymMGck
I can see you are using the old version of WordPress, so please update it as well.
And the Notices you are getting on your site are from the Elementor plugin not from our theme, and those will not harm your site, so please ignore them and turn OFF your site debug mode and it will hide those from your site.
Regards 8Themes Team.