Revised for Arduino version 1.6.6+, not tested with earlier versions.
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####INSTALLATION
Look in ~/Applications/Arduino.app/Contents/Java/lib and replace the theme folder inside (making a copy of the original in case want to revert back).
####CREATING YOUR OWN MODS
The newest version of the Arduino IDE makes creating custom themes trickier: you now need to edit the theme.txt file, an XML file inside the syntax folder, and the button files. Unfortunately, not all items in the theme.txt file actually work, so if you can't get an item to change, try another one of the files.
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Released under Creative Commons BY-NC-SA license - feel free to use but please let me know.