>it is but it's not feasible for the system theme to change the styles of websites.
Just ask any Linux user that uses dark themes and you will find out what happens when the system theme messes with the site theme.
It is done completely different fashion than it was done on gnu/linux, on linux it was probably done by changing user agent style sheet (changes default bg color on body etc), when here it is done with media query (which webdev must explicit implement on his part so he can make sure that everything works).