Ideally, I'd like to be able to include the modules and see no immediate effect — completely opt-in. I could first opt-in to the basic styling via a class on the body tag. Then I could opt-in to other components via classes and specific structures. Namespacing the CSS classes would be great, too, so there's little chance of colliding with my custom CSS [first-party, first-class] classes [that I shouldn't have to namespace].
The web UI kit I'm using now that comes closest to this ideal for me is https://getuikit.com/.