I just mostly hate dark themes, they tend to scream unprofessional. At the same time if it's a well designed theme, having a dark background won't be a bad thing because the designer is intelligent enough to make the entire theme complimentary and not blinding.

Anyway, what I'd say is just set whatever theme you want to be the default (if you can find one you like) but allow registered users to change the theme to the default at least. I don't particularly like the default, but it'd be much easier on the eyes than something with a black background. Plus, being the official theme, you wouldn't have to support anything extra -> no maintenance cost.