A very simple dark mode implementation based on a single css attribute "mix-blend-mode".
It is small, simple and easy to use: just click on extension to enable dark mode and click again to switch it off. Your choice is retained across all websites you activate the extension for.
It works well with almost any website or pdf-document, but images may look like negatives due to color inversion.
The extension is free and open-source, and you are welcome to improve it!