OneNord is a clean and vibrant theme that combines the Nord and Atom One Dark color palettes.
Download OneNord.xccolortheme and move it to the user theme directory:
mv OneNord.xccolortheme ~/Library/Developer/Xcode/UserData/FontAndColorThemes/
Note
If the folder doesn't exist, you will have to create it:
mkdir -p ~/Library/Developer/Xcode/UserData/FontAndColorThemes