I always wanted a darker version of gmail, guess my wish is fulfilled now. :) For the same thing earlier I had installed greasemonkey, but now its right within Gmail. Thank you Team Gmail. But still the old theme wasn't all that bad. It was the simplicity of the interface which made Gmail really attractive & user friendly. But having a themes is not all that bad either. And not only that all the 30 themes are really good.
Image Credit: http://googlesystem.blogspot.com/