Add ability to switch mode based on Battery saver on/off status

For my Android and MacOS I've set the device up to change to dark mode when battery saver is enabled/battery level goes under 30% and change back to light mode when battery is above 30%. I'd like for this to be an option in the app.

Doing this makes it really obvious that device needs to be connected to power soon.

The issue names no files, tests, or entry points. Start by locating the app's existing automatic theme-switching options and how it observes Windows battery-saver or battery-level state; done means the app can switch to dark mode when battery saver is enabled or the level is below 30%, then return to light mode when the level rises above 30%.

Written by the indexing model from the issue text.