You must create a `RailsCasts` theme inside IntelliJ first. Do this in Settings > Editor > Colors & Fonts,
select any existing scheme and click "Save As" and name the new scheme `RailsCasts`

Close IntelliJ, then go to `~/Library/Preferences/IntelliJIdeaXXX/colors` (where 'XXX' is the current version of IntelliJ
you have installed) and there should be a `RailsCasts.icls` file which can be replaced with this one.