I don't think you currently have that option in the stock email client (although I think I read that this might be coming in a software update). From what I have read here, the stock email client background is white for some (all?) the U.S. carriers, and black for carriers outside of the U.S. (at least in the UK, not sure about other countries). AFAIK there is currently no way to change this and set a dark theme, at least not that I have found on my phone (VZ)
I know that at least some other email clients support dark themes. I'm using a dark theme with Touchdown right now, and I assume that the other major clients like K9 and Aqua Mail probably have this option as well, but I haven't used them yet, so I don't know for sure.
