Incredibly useful, I often work at night and this add-ons make all my webpages in dark mode, the colors are generally good on every website, I use it also when in Wordpress admin, I use a plugin like "Admin Color Schemes" for the left side bar and top bar, the rest is darker with the use of this addon. I can be used not only with website but also with "web apps" like phpMyAdmin or any kind of that opens in the browser, making them dark mode even if the don't provide dark mode