How to test preferred color scheme (Dark Mode) with Firefox

As big fan of dark modes, I was very happy to learn that Firefox now supports the prefers-color-scheme media query.

However, I find i a bit hard to test this quickly on my maschine. It would be great to be able to switch between prefered color schemes similarily to the device presets in responsive mode. Or to be able to set a prefered scheme using developer tools.

I think this might be related to:

You can check this addon that might help you:

For me, the developer version on Firefox stays on light mode even if the system is on dark. It works perfectly on stable Firefox and Chrome. what could be going on?