Still with multiple versions of mame possible in the next mgalaxy, a vertical romset and a separate horizontal set could accompany a true UI rotation nicely.
I'm planning to move the theme choice to the 'system' tab.
In clear, that mean that you'll decide which theme to run for each 'system'.
So, it will perfectly answer the situation that you're describing.
For that occasion, I will create a 'true' (as you call it) theme!
I hope that this update project will satisfy your expectation!
(now, it's just a question of time to implement it...I can't affirm you that it will be in the next release)