amoled theme (pure black) when auto bright-dark is used

[AnthillSudoku]

If you customize Fossify colours you can make a pure black background for amoled display.

The same is not true with auto bright - dark, where you get a dark graish look.

Ideas to have in in the UI could be:

• add "auto bright / pure black" to the list
• or add a toggle "pure black mode" alongside the colour page

[Aga-C]

It's already requested here: #109.