The T-Deck firmware v9.4 now has a new Display preference which lets you have a Light or Dark map theme, and you don't need to have two tile sets. It simply darkens the existing tiles.
The Preference
If you go to the Display settings screen, there is now a new Map Theme preference:
This can be set to: Light, Dark or Auto. The Auto setting is really handy, and uses the current Brightness setting to determine Light or Dark mode. (ie. if you have brightness set to Day, it uses Light map theme)
This preference will also be coming soon in the other firmwares, like the T-Pager, T-Watch S3 Plus, etc.