I've been using a Monokai theme in Sublime Text 2 for a while and I really like it. It's easy on my eyes.
This Xcode 4 theme is ported from Joseph McLaughlin's theme for Xcode 3. All I did was convert it to the new Xcode 4 format. Joseph's theme was a port of the Textmate Monokai theme.
To install, copy the file to ~/Library/Developer/Xcode/UserData/FontAndColorThemes and restart Xcode.