Dark/white. Compact/expanded. Color variations. Palette variations. Ever heard of white label websites too?

Imagine it more from the perspective of a component altering it's style based on some sort of global "state". That global state could simply be a class on the body like "page-contact", so your css can alter itself much easier if the style isn't defined up front in a list of prescriptive class names.

It's less theming in the sense of css zen garden and more encoding variations of style in css instead of relying on more class names.