It’s fine if the user actively chooses between the sub’s style, the site’s style, or even a custom personal style. However, I think that it’s important for communities to have at least some room to customise their own look-and-feel - it delivers the point better, that Reddit is supposed to be an aggregate of communities, not some huge monolithic community on its own.

And custom CSS in special was the target of protests, as Reddit already tried to remove it.