Black/dark mode

[rugk]

Already mentioned in #183 (comment), but an issue for that was still missing…

Note:

• a black mode should also be suitable for AMOLED users wanting #000 as much as possible. This may even have a strong contrast so that these users can use the app at daytime with the screen always-on, but can easily see what matters; thus maybe explicitly call it "AMOLED mode"
• the usual dark mode can be a mode with just some dark grey things and low contrast to be used at night, maybe call it "night mode"

[westnordost]

When a custom style is implemented, the color values can be defined at the start as constants and be exchanged. In other words: technically it is possible. but it is blocked by #183. I won't get to that anytime soon but as said, you are invited to busy yourself with that.

[westnordost]

Blocked by #183

[westnordost]

This black mode should make use of the DayNight base themes, see
https://blog.iamsuleiman.com/daynight-theme-android-tutorial-example/
and use the dark-blueish map style.