Dark Reader is the best extention for this, pretty sure it's open sourced too!

Dark reader for firefox

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.

Dark Mode for Firefox is decent.

It did break the Dropbox website for me though. Had to disable the extension to get the website to work.

I've used Dark Background and Light Text and it seems to be faster than Dark Reader but I like them both

There's also Midnight Lizard. It's more powerful, but more resource intensive so I wouldn't recommend on phones or older systems.