First get a dark theme, then get a .onion address and finally expand the forum to include a 'malware creation' section alongside malware removal and you're well on your way to attracting the 'underground' crowd
Perhaps a theme with some darker greys would work, black is perhaps a bit too much but grey instead of white is certainly easier on the eyes. Having never used Xenforo I don't know if there's some kind of theme switcher you can include, that could be a solution perhaps? ...Or maybe I should just write a browser plugin to darken the theme for any users that want it