Try this search: https://addons.mozilla.org/en-US/firefox/search/?q=dark&type=extension

Usually addons will transform each page with a custom CSS to make it dark mode. If you are talking about the native dark mode of each website, then I don't think there is a simple answer to this. Because each website is solving this differently. So we rely on custom CSS to force parts of the website styling in a temporary manner, each time it is opened up.