Implement auto dark mode

Problem/Motivation

The user can currently pick light or dark mode.
Many modern frontends now allow to have the "system" option, getting the settings from the current system.
See https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-sc...

Steps to reproduce

Proposed resolution

Adjust the CSS and body class in order to support "prefers-color-scheme".
This will allow to inherit the settings from the system.

Remaining tasks

User interface changes

API changes

Data model changes