The map should be dark in dark theme
The map in Gnome Maps is always bright, which kinda defeats the purpose of using dark theme. Other parts of the application are dark, but the main part isn't.
I believe this is because OpenStreetMap doesn't provide dark map tiles and the map we receive from them is always bright.
Possible Workaround
I don't know how Gnome apps work, but I noticed this app uses CSS and some time ago I created a CSS workaround for this exact problem. I use it successfully in my web app ISS Viewer and perhaps it could work here too? You would just need to apply this CSS to the map element:
filter: brightness(0.6) invert(1) contrast(3) hue-rotate(200deg) saturate(0.3) brightness(0.7);
Result:
If this can work, it would be nice to have it applied in dark theme by default, but a setting would be fine too.