I'm a bit confused with this idea. Would it not make more sense to merely disable the promotion option for those accounts that were created before the date of implementation? That way if the account has option to promote, they are strictly (relatively) new.
The customisation bit isn't out of the question, though. An extension like Stylish allows you to choose the details like fonts from a menu (and I think that there's a convenient spectrum to pick the colours so that you don't have to speak hexadecimal to set them). I can try and cook something like that and post it on the Userstyles page later this week.
On the side of kb, pretty sure he thinks dark mode is dirty enough, if ya smell what I'm farting.