Nope this ideally needs to be done in the ACP for now, in the past 5 months I've managed to get 31 admins to transfer from a specific self hosted platform to self hosting their own phBB boards, the reason they were originally on the self hosted platforms before they were brought by another company was because they know little about mysql, setting up, coding and other nitty gritty stuff.
So the aim has been to get them all set up and started and along with the custom style a simple extension that lets them hide/display the site logo/site name on big/small screens, choose if or not to hide the index login, change the basic style colours to blend with their forum subject.
Am sure you agree that if someone is paying you for something you want to make it as easy as possible for them to use. (I just thought it would be cushty to make it stop adjusting the Prosilver style incase the admin wants to also allow members to choose between custom style or Prosilver)
I agree that everyone should have some basic knowledge of phpBB, mysql and coding so they can apply fixes to files after updates etc and keep their boards in shape... and for this - everyone one of those forums has been set up with 2 spare test forums on sub domains so the admins can continue learning what's what while their main forum is up and running.
For now i've added .custom in front of the custom styles classes so that even though Prosilver loads the sheet - the changes don't affect anything in the Prosilver style - so for now it's sorted.
Always encouraging people to take back control and not be held to ransom.
Often found lurking over at TapaTalk