It's not about forcing to change to a one-and-only theme of their choosing. We already know what their choosing is (bad) It's about giving people the choice they want for themselves. Four themes to choose:
Nostalghia theme (current green goo shit)
Light theme (white/light gray)
Gray theme (50% gray)
Dark theme (black/dark gray)
Or if it's easier == allow 0-100% fine adjustment, like in Windows and most graphics/photo programs.
I would suggest a dark grey. It's best as background for pictures. Look for example on Flickr. Alternatively, I would opt for white, which IMHO is the best alternative for dark grey. BTW, the grey should not have a slight colouring in it.
Hm, that's a really good question.... In a number of way, including color psychology and supporting the colors of any and all art that's posted here, DeviantArt would likely be better off sticking more or less with the current color scheme. As far as my own tastes go though, I would find a darker theme to be far more appealing, even if that could risk somewhat overshadowing darker pieces if it's executed poorly.
I'd had to see different executions to know for sure what I'd think of them, but for now I've voted for the darker theme.