It's not an unreasonable expectation for developers to have the basic skills needed for the profession. It's completely nuts to build for people who expect to make their living using a library but "abstract" away the basic skills you should be able to assume they have. I actually hate libraries like Material UI because they use generated classes that make it very difficult to override their default styling (and inevitably I'm going to get a comp from a designer who has no idea how the library works that means I need to override it).

If you lack CSS skills, you almost certainly are missing other essential skills, like knowing how to write semantic HTML and make things accessible. Please learn your trade--you have a large audience. Normalizing the idea that you can do this professionally without understanding the skillset people hire us for is not ok.