Firefox: How to test prefers-color-scheme?

Question

In Firefox 67, one can use media queries to detect user preference on light or dark theme.

In my version of Firefox (under Ubuntu), it seems that my preference is light

Answer 1

Just for the sake of completeness: If everything fails, there is a Firefox add-on called “Dark Website Forcer” (also on GitHub).
It changes the style by reading the CSS of the website and re-applying the parts for the dark version again, if you want to have a dark one. As such, it also can only force a dark website, not the light one. The Readme has a detailed explanation why.

Disclosure: I'm the author of this add-on.