Railscasts_xcode_color_theme is a project mainly written in ..., it's free.
Railscasts Xcode color theme
Xcode 3:
mkdir -p ~/Library/Application Support/Xcode/Color Themes cp Xcode3/Railscasts.xccolortheme ~/Library/Application Support/Xcode/Color Themes
Xcode 4:
mkdir -p ~/Library/Developer/Xcode/UserData/FontAndColorThemes cp Xcode4/Railscasts.dvtcolortheme ~/Library/Developer/Xcode/UserData/FontAndColorThemes
Based on Ryan Bates' Railscasts Textmate theme