The default white is very tiring for the eye after some hours.
I created own dark theme for Mathematica. It looks like
All stylesheets needed for that you can found here [repo]=https://github.com/dmarienko/Matlab/tree/master/.Mathematica You need to copy .nb files from repo [repo]/SystemFiles/FrontEnd/StyleSheets/ to your local Mathematica StyleSheets folder. After that you can choose stylesheet named as Dimad.nb (You also may set it as default stylesheet). Also there is Plot's functions color palette for dark theme in [repo]/Kernel/init.m. You need to add it's content to your local .Mathematica/Kernel/init.m file. Hope it helps.
PS: All was done for Mathematica 9.0, I didn't test it for other versions.
PPS: It uses Ubuntu Mono as default font. It can be obtained here http://font.ubuntu.com/
The quickest thing might be to modify the default stylesheet and save it as a new one. You can find a good description of how to do that on David Park's Mathematica page http://home.comcast.net/~djmpark/Mathematica.html.
If you want a ready-made stylesheet, I've found David Park's