With this implementation of the feature of the automatic theme
detection, we make the following changes in the backend, frontend and
documentation.
This replaces the previous night_mode boolean with an enum, with the
default value being to use the prefers-color-scheme feature of the
operating system to determine which theme to use.
Fixes: #14451.
Co-authored-by: @kPerikou <44238834+kPerikou@users.noreply.github.com>