Minor: simplify the themes

[richtea999]

Very minor, but whilst I'm thinking about it (having been surprised by a dark map on a bright day)...

There are 3 references to 2 theme features:

• App theme in Account Menu
• App theme in Navigation settings (identical to above?)
• Map theme in Navigation settings -> Map layer

I would think people who want a dark app theme also want a dark map theme to match (and they'll want both to be 'automatic' to follow the device theme).

Maybe they could be combined into a single setting?

[Corjan Meijerink]

@richtea999 Future ambition!
First two are duplicates indeed as this provides easy access when opening the Navigation settings while navigating (other settings cannot be accessed).

Further explanation can be found in this other recent post: https://forum.myrouteapp.com/topic/4507/app-theme