Hehe, I just realized I could, theoretically, use MySQL views to avoid doing ANY data-oriented theme adjustments. The idea would be to create a BMW view that shows only posts and pages with category ‘BMW’, and another MINI view that shows only posts and pages with category MINI. I cannot crate cross-db views, so I would have to play with the wordpress prefix (e.g. instead of wp_ have bmw_, mini_)
I would still have to modify the theme to do css selection, though. ??