Example of this: https://www.theglobeandmail.com/

The extension takes effect and then some more JS is loaded that resets the site. It's funny, I actually was looking for this kind of extension because of that particular news site.

You can see in the Performance tab that there's a chunk of time where the extension works:

And then it's reset:

Also, some of the dark mode is not reset, there's a text area comment that correctly shows up.

One hacky solution would be to allow re-applying the styles a few seconds after page load, based on a per-website setting.