Feature request: add dark mode

[eUgEntOptIc44]

Hi @Pygmalion69

I'm using your great app for a few years. And I really enjoy it daily 👍

However the addition of a (optional) dark mode would be great to further improve the UX. osmdroid has a (basic) support for dark map tiles included. Please also see here: https://github.com/osmdroid/osmdroid/issues/1804#issuecomment-1128923695[^1]

Also the ability to toggle the visibility of the app bar as requested in #21 would be such a huge improvement for devices featuring a small screen.

If you're interested in (further) translations of the app please let my know. I'd be happy to support you.

[^1]: Their using a ColorMatrixColorFilter for this see: https://github.com/osmdroid/osmdroid/blob/dbb20901cac30b28a6a9349da097c22d9a962311/osmdroid-android/src/main/java/org/osmdroid/views/overlay/TilesOverlay.java#L95