Feature: Allow choosing "Black" theme as replacement for "Dark" when on "Auto"

### Your use case

#### What would you like to do?

When the theme is set to "Auto", the app follows the device's system theme. It would be a nice addition, to have the option, to use the Black theme instead of Dark there.

#### Why would you like to do it?

Possibly lower power consumption and better eye comfort when using Dark theme for night mode.

#### How would you like to achieve it?

Introduce a new setting for it and when on Auto, instead of statically changing to Dark theme, check which one is preferred by the user and change accordingly.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating the theme settings and the Auto-mode logic, then trace how Auto currently selects Dark. Done means users can configure Black as the Auto fallback and the selection is applied consistently on iOS.

Written by the indexing model from the issue text.