I think that something important to consider is implications if Core includes an extender system for themes (the way I see this working is that it's extremely similar to the frontend extender, but is dependent on the theme being enabled, and one one theme can be enabled at a time). There's a lot of things to consider (eg. would color palette selection and dark mode be independent of theme), but extensions like this could serve as a way to test out possible UI systems for controlling more abstract themes in the meantime.