As much fondness as I have for the pure separation-of-concerns CSS Zen Garden style, I have to admit I've yet to actually see it leveraged on a real project. In 20 years of working on the Web. I'm not sure I've ever seen a significant re-working of CSS to re-theme a site without also changing the markup substantially.
In practice, maybe coupling the two doesn't actually do much harm, provided consistency of approach is maintained and it doesn't go quite as far as inlining CSS all over the place.
