Save it into
~/Library/Application Support/Xcode/Color Themes/, restart Xcode, and open its Preferences. You will be able to pick the Railscasts Color Theme in the Fonts & Colors tab.
I dug this theme up after realizing that I haven't actually saved the Railscasts theme I'd replicated from several months ago and was too lazy to set up another one in my other Mac.
Koen Van Der Auwera did a similar theme too so if you find that mine doesn't work so well (I've only used it for Cocoa Touch development, not at all for developing Mac apps), scoot over to his blog post.