Nice idea, but the option to change theme based on system doesn't work for me. To be more accurate, it works inconsistently: sometimes it changes it, sometimes it doesn't.

Works, more or less. I'm using the "sync with system" setting and sometimes, the proper theme does not get applied. Not sure why, I couldn't find a pattern, and it's a shame because the idea is good (I'm surprised Firefox does not have such a setting built-in)

When selecting "Change themes based on the system theme", the extension doesn't change the theme automatically.
I have to manually select another option, and select back again "based on the system theme" for the theme to take effect.

It's the only extension of it's type so I'm glad it exists, but a large percentage of the time when I log into my computer, it fails to switch from light to dark or vice versa. Wish it worked better, but glad it exists.