You can now override base game assets officially using an override mod.
This can be used for total conversion mods, like changing out all base game art and audio for those from another game.
You could even use it to experiment with custom themes, although it might be better to wait for proper theme modding.

You can replace textures, audio files, and languages!
This works for any mod by the way, not just the base Quest Master mod.
Note that since they only override assets, these mods won't be bundled automatically with dungeons you upload. This is by design, since there will be total conversion mods you want to be able to swap out.
If you want to force the mod to be bundled with your dungeon, one way you could achieve that is add a dummy item to the mod and add that into your dungeon. The game will then detect the item, identify its mod, and bundle it to your dungeon.
Otherwise you could also simply recommend the user a specific mod in your dungeon description for "best experience".
Also note that since we are still in Early Access, files are expected to be moved around. So in the near future, you might have to update your override mods accordingly to fix relative asset locations.