Draft: Fix edge cases hit with "Follow system theme"

panic: Use standard preferences

• These custom preferences don't play well with all theme combinations,

  F-Droid theme:

  • light -> light red, all good.
  • dark -> dark red, all good.
  • follow system, system light -> dark red, BAD.
  • follow system, system dark -> dark red, good.

• This when combined with other accessibility options (such as monochromacy) led to an entirely black checkbox, which is not good.

• Rather than try and think of and cover all scenarios, it might be easier to switch to standard preferences, and if needed, figure out other ways of emphasis.

Also deprecate isAppThemeLight() since there was just one more user of that.