Currently car themes and restrictions apply to both the player and drivatars. Meaning if I build a muscle car for dirt racing, the drivatars will also use muscle cars by default. However if I wanted the AI to specifically use Rally Monsters cars in the dirt race while the player can pick whichever car they want a la Anything Goes, that cannot be done.
Another example I can think of is having an event at the Goliath where the drivatars use S2 extreme track toys, however the player is free to choose whatever car they like for better or worse. Obviously how good the AI will be will still ultimately depend on the set difficulty, but I think having the option to choose which cars the drivatars use separately from the player would greatly expand the opponent car variety and open up the possibility for more challenging custom races.