I'm still fine-tuning, but I have added a couple of options.
Once you've selected an option, it will stay with you until you change it again, or for a year if you don't change it. I may add other combinations later.
Hmm. I can't help thinking that this overly complicates things - after all, the theme will revert back when the cookie expires, and I would have to reset it whenever I log on at a different computer.
SMF▸ has a built in themes feature; why not just use that?