Dark theme

[Korb]

As the screenshots provided on the app's F-Droid page suggest, it does not support night color mode. Since the README.md also does not say anything about the dark theme,

I suggest adding it, because at night the light screen is very straining for the eyes and distracts from the road.

[boldtrn]

Once graphhopper-maps supports a dark theme, we could introduce it here as well. That said, I am not sure if this is out of scope?

[karussell]

Have thought about it already: graphhopper/graphhopper-maps#308

Maybe someone could help :)