This is an experiment for different ways to activate dark mode.
In order of relevance/final impact those are:
When the user does an active selection of a theme with the buttons, it is persisted and used as the default setting for next website visit.
If there isn't a previous setting stored then it checks for the OS dark theme option to see if it is enabled.
If there are no settings to default to, then it sets the dark mode when it is night time (21hs-7hs).