I use a lot my IDEs during everyday’s work, so I try to minimize the strain done to my eyes. And I find it more comfortable if I use dark backgrounds, hence my themes.
If you want to try, run this on a terminal with XCode closed.
cd ~/Library/Fonts; curl -O http://levien.com/type/myfonts/Inconsolata.otf; mkdir -p ~/Library/Developer/Xcode/UserData/FontAndColorThemes; cd ~/Library/Developer/Xcode/UserData/FontAndColorThemes; curl -O https://raw.github.com/gist/3427070/70427af513bc71b011ac65f847435fbe169d2170/EGOv2_Inconsolata.dvtcolortheme
And then activate it in Preferences / Fonts & Colors . Regards!