ThemeService: add a 'Auto (Black)' theme option

[tamcore]

Closes #5966 and #6959.

Pull Request Checklist

• I read the contributing guide
• UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• Accessibility has been taken into account.
• Pull request is based on the develop branch
• Pull request contains a changelog file in ./changelog.d
• You've made a self review of your PR
• Pull request includes screenshots or videos of UI changes
  • https://user-images.githubusercontent.com/319917/200836824-2a7000e3-3179-4c0f-995d-5f16ad9100df.mp4
• Pull request includes a sign off

[BerndSchmecka]

Can you request a review?

[tamcore]

@BerndSchmecka unfortunately don't have any buttons or whatsoever for that. Also asked in the Matrix channel but didn't get any feedback :(

[BerndSchmecka]

Oh :( Hopefully someone with write access will find this PR and review it so that everyone can enjoy the black theme only at night ^^

[daniellekirkwood]

Approved from Product perspective.

[CLAassistant]

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