- Go to the finder.
- Go to the navigation bar, click the "Go" button while hosting the option key and go the library.
- Go to Developer -> Xcode -> UserData -> FontsAndColorThemes and add the xccolortheme file to the FontsAndColorThemes folder.
If you are unable to see the FontsAndColorThemes, go ahead and make a folder in UserData folder called "FontsAndColorThemes". You can use the following command in terminal.
$ mkdir -p ~/Library/Developer/Xcode/UserData/FontAndColorThemes/
$ cp path/to/theme/*.xccolortheme ~/Library/Developer/Xcode/UserData/FontAndColorThemes/
Check the FontAndColorThemes directory for examples. You can modify after adding them to your disk. Check the Xcode Settings > Themes.