Put twilighty.xml in ~/Library/Preferences/RubyMine10/colors
Restart RubyMine and then set your color scheme by going to File -> Settings -> Editor -> Colors & Fonts, choose a color scheme from the dropdown box, and click Apply
1.1 - Better highlighting of stuff
"I'm not a dark background guy, but if I were, I'd use this."
-Adam M.
"Hey what's that theme?"
-David S.
Enjoy!