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/
Format -> Stylesheet -> ReverseColor:
I don't know how about you but I missed the fact it is built-in since V 10.
I didn't like to coloring in ReverseColor much so I patched it a little and added to my https://github.com/kubaPod/DevTools
I was focusing on making it neat for .m files rather than .nb so it may look weird in place of default styles.
See https://mathematica.stackexchange.com/a/164654/5478 or Github readme for installation steps.
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
PresentationsStyle stylesheet especially easy on the eyes. It not only uses a light brownish background but changes other styles so as to harmonize. If you own Park's Presentations add-on, then you already have this stylesheet. If you don't, you can obtain it as part of his newly-available FreePresentations add-on (which allows you to read documents prepared with the full Presentations but not conveniently to author new ones):