GitHub (Light) and GitHub (Dark) are Xcode themes based on themes of the same name from GitHub for Visual Studio Code.
To install, create a directory at ~/Library/Developer/Xcode/UserData/FontAndColorThemes/ if it does not already exist and copy GitHub (Light).xccolortheme and GitHub (Dark).xccolortheme into that directory.
Or use these Bash commands:
mkdir ~/Library/Developer/Xcode/UserData/FontAndColorThemes/
cp "GitHub (Light).xccolortheme" ~/Library/Developer/Xcode/UserData/FontAndColorThemes/
cp "GitHub (Dark).xccolortheme" ~/Library/Developer/Xcode/UserData/FontAndColorThemes/
