Case in point, actually. I've done several multi variant themes, for 1.1.x and 2.0.x, and the variant system brings in problems.
Say you want to create a dark variant and a light one. This means the best text colours are often going to be different. For example, if you have one membergroup coloured purple it may be perfectly fine on the light variant, but almost illegible on the dark one. If you have a group coloured a pale yellow it may be fine on the dark variant but not on the light one.
Then there's the background colours themselves, regardless of lightness or darkness. Some colours just don't go well together, and the more adventurous you get with variants the more you'll find yourself wishing that some dumb motherf***ers hadn't forced everything to be one way in Sources.

You'll find yourself long for things like group colours assignable by theme and by variant. You might even think about writing a mod for it, when it should have been doable in default.
