I have firefox 67 install on f30 and its not honouring the prefers-color-scheme CCS media
I raise https://bugzilla.redhat.com/show_bug.cgi?id=1716134
but no useful feedback yet.
It explains how to check the CCS medai query.
I use KDE in dark mode by using the Breeze Dark theme.
I was told that the problem might be a bug in the Breeze Dark theme if it does not
set the "dark mode" in the right way.