Wonderful documentation, M can eat his words about how the devs don't help modders now
Instead of there being a hard-coded "custom" gamemode, you should be able to take any gamemode and click "Customize", giving you the same options but based on that gamemode and with that gamemode's name showing up to the scripts. So if you, say, wanted to use Freebuild with some extra brick packs you wouldn't need to read through the gamemode file to get the list of default add-ons, not to even mention colorsets.