Hello, @Bas Kling,
Actually that was not the specific CSS code, but the general one, and that is the reason it applies on the complete site popup, If you want to set the CSS for any specific popup then you have to set/choose the specific Custom CSS for it.
Hope this makes sense, If you are unable to then please share the screenshot and URL of that popup so that I will provide you the Custom CSS fix for it specifically.
Regards 8Themes Team.