I suppose that depends on what type of user you're targeting. I'm a front-end engineer, not a designer, so I want to be able to smoothly integrate with tools with minimal conflicts. I don't think it's asking too much of a user to add a class to the body. Consider that even if you're targeting non-programmers (designers), they will still customize the UI on top of your library and therefore know how to add a class to a tag.

On the other hand, I've tried somewhat- or very-opinionated UI kits and if there was something I didn't like or want to use, I had to find a way to opt-out, which was always ugly and not sustainable.

Additionally, this helps semantics and readability, and helps keep things scoped. What if I want to use your UI kit in a subsection of my document but not the whole thing?

To me, that is the holy grail of UI kits.