I don't like this because the whole theming situation is already pretty terrible in my opinion and adding yet another layer of settings is just over-engineering the whole thing. How about Plone just uses default Bootstrap to begin with, there's an idea...
...and really, the in-line theme editor is just so... What. I mean I get that we're wanting to make the whole thing more accessible to designers and frontend developers but it just makes no sense in the first place since the amount of people trying to theme Plone as designers is from what I can see, pretty negligible. I guess that's why you can find loads of Django themes and well, basically none for Plone. And the custom themes that are there for Plone aren't up to snuff when you consider what's already out there.
Not to rant, but I definitely would like to see some simplification of the theming system, Diazo and Barceloneta are all great and well, but they become incredibly tenuous to handle when you start REALLY trying to develop a custom UI or application styling.
For example I'd love to implement https://getuikit.com/ into a Plone application, but where would you even start before having to fix stuff before creating things. It should be the other way around.
As I see it you can either use Barceloneta and re-theme the site/application or if you want to create a custom theme and decide to disable Barceloneta then you're thrown in the wild to have to re-style pretty much the entire interface and since Barceloneta only uses derivatives of Bootstrap, but not Bootstrap class names or styling, and since most of the Plone templates don't use defaults for classes but custom ids here and there, you basically end up with a broken interface.