I say we vote on which one is the widely accepted "main" theme, have an overall discussion based on that, then provide access to both themes, knowing beforehand that the other theme is not necessarily the "supported theme". That would be a fair way to handle it I think. Everyone gets to choose which one they like the most and then you only have to work on one theme as opposed to two all of the time. Maybe slight tweaks would be made to the unsupported theme here and there, but it would be widely known and accepted that it wouldn't happen often if ever, mainly since that would help keep you sane, lol.
I think that's a fair way to do it, really. Helps everyone out and everyone gets what they want at least for a short while. If you use the theme that most of us aren't using, then you do so with the knowledge of that, so things won't seem to all fit together, but that's fine since you made the decision yourself and aren't forced to do anything.
Just as a preemptive vote here, I want the "dark" theme to be our "main" and "supported" theme. Reminds me of URealms to some extent (darker forums all remind me of URealms just because nearly every other site from 2007 and onward use a lighter set of colors than URealms ever really did, at least on the forums) and allows for actual font colors to be used (I like the fact that Tirin and 13th get to use their traditional colors and that I can still read them all of the time with ease). The "light" theme doesn't really allow that to happen. Besides, darker themed forums are easier on the eyes, imho. I'm a cave dwelling fairy tale creature, I don't like light.