Some websites (like this one) now offers two differents themes: light and dark.
Dark themes are proven to be better for the eyes and reduce battery usage on
mobiles devices because it requires less light to be displayed hence it
requires less energy to display. The gain is optimal on OLED devices but it
also works on classic LCD screens.
While on Windows and MacOS there is a global setting for the user interface in
which you choose if your system is in light or dark mode, with that setting
being used by lot of applications supporting dark/light themes, on Linux and
BSDs (and others) operating systems there is no such settings and your web
browser will keep displaying the light theme all the time.
Hopefully, it can be fixed in firefox as as explained in the
documentation.
To make it short, in the about:config special Firefox page, one can create a
new key ui.systemUsesDarkTheme with a number value of 1, the firefox
about:config page should turn dark immediately and then Firefox will try to use
dark themes when they are available.
You should note that as explained in the mozilla documentation, if you have the
key privacy.resistFingerprinting set to true the dark mode can’t be used.
It seems dark mode and privacy can’t belong together for some reasons.
Many thanks to https://tilde.zone/@andinus who
pointed me this out after I overlooked that page and searched a long time with
no result how to make Firefox display website using the dark theme.