I have R20 of Broadcast. In Preferences, Under Interface, one can set the scheme to Dark or Light. I also though adjusted some of the Scheme Colors settings to match Dark Mode under macOS a bit better.
However, I don’t see a method of saving such changes as a custom scheme. Sometimes it’s nice to be able to switch between a custom preset and back to the original factory settings. I can switch back to the factory settings, but that involves deleting files in preferences.