Xcode keeps themes in a folder: ~/Library/Developer/Xcode/UserData/FontAndColorThemes so here are my recommended steps to add this theme to your Xcode:
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1) Open Xcode
2) Choose Xcode > Preferences
3) Choose Fonts & Colors
4) Press the + button in the bottom left corner and make a new theme named "Mulder"
5) Quit Xcode
6) Copy the downloaded Mulder.dvtcolortheme file to ~/Library/Developer/Xcode/UserData/FontAndColorThemes/.
7) Open Xcode and enjoy a greener coding experience. :-)