When tweaking the dark-mode of my site, I frequently change the ui.systemUsesDarkTheme in about:config to see how it looks. However, this involves changing the setting, switching to my site and reloading it. But there is a handy extension by rugk that makes this very easy.

It modifies the output of media queries which looks like wizardry to me 😅

Note: It only works on websites that support the prefers-color-scheme query.