Browser Extension for Web Developers to Test Dark CSS

To test your own prefers-color-scheme styles, use Chrome DevTools rather than an extension.

Open the Command Menu (Ctrl+Shift+P, or Cmd+Shift+P on a Mac), run "Show Rendering", and set "Emulate CSS media feature prefers-color-scheme" to dark. That shows exactly what visitors with a dark system theme see. Modern Dark Mode does something different: it forces dark colors onto a site with a CSS filter, so it's useful for seeing how your site looks to people who use a filter-style dark mode extension, but not for testing your own dark theme. Exclude your site in its popup while testing, or it will flip your dark theme back to light.