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.

I myself always set a light theme in my Firefox config so I don't get a black on black text/background issue.

With how customized the styling on sites is every site needs to implement their own dark version otherwise you will run into situations where the styling is broken.