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
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
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.