Support automatic Light/Dark mode, e.g. via Theme.AppCompat.DayNight

Idea

• Automatically switch between Light and Dark theme variants
  • Time of day
  • Using Theme.AppCompat.DayNight to follow system-wide theme, e.g. as per the Android 10 developer documentation and LineageOS' Styles API

Rationale

Dark theme can easier on the eyes at night, but the light theme is better for reading in daylight/sunlight. Manually switching works, but can be a slight hassle.

Time-based is sufficient, though integrating with the system-wide choice on devices that support it makes for a smoother experience by having everything follow a single setting.

Additional

This is now available in Android 10, hurrah!

The LineageOS' Styles API also has the option for setting an accent color without needing the Lineage SDK. Might be neat to integrate this somehow.