Your customisation options are great, but still, I can't get a good result with my brand colours. It would be amazing if you allowed custom CSS. Sure, this is an option for advanced users, but I think a warning popping up when turning it on and a quick way to turn it off will do the trick. You might be worried about support tickets, but you can always say that you are not supporting the users’ input there and have a stock reply that they can easily flip the switch off if they don't know what they are doing :)