If you're looking for an addon that allows you to control which website gets dark shaded, this one will do it for you. But it is better than other dark themed addon because you can customize a lot of options, including changing how it colored the text and links to your liking.

Most powerful, you can exclude websites from this addon, so sites with built-in dark themes won't be affected by this addon. The addon's dark mode not only changes the webpage, but also the interface to match. Don't let the fact it has few reviews scare you, it's really the best dark addon I have found so far.