Feat: Add setting to switch between dark/light/system

[escapedcat]

I'm using dark mode as a default but at least for me all maps are hard to read in dark mode. So I tend to switch "map-ish apps" to light mode.
Would like to have the option for btcmap as well.

[dadofsambonzuki]

Agree. Default should be system (as it is currently) with ability to override.

[bubelov]

It requires too many hacks so this approach won't work well.

What we can do instead is to add a map style selector. We already have more than 5 themes:

https://openfreemap.org/

• Positron
• Bright
• Liberty
• 3D
• Our custom dark theme

This selector is applicable to the map widget alone, so the rest of the app UI would stay system-default. It can cause minor contrast issues with the status bar, but otherwise it should work fine I think.

We can also allow users to set their own tile server/theme URL so they can use other styles or even self-hosted maps.