Comment # 3 on bug 1023579 from Atri Bhattacharya

The ability to set a "Global dark theme" has been removed from gnome-tweak-tool
(v 3.30). Setting the theme to "Adwaita dark" from the same tool also applies
it correctly for firefox (if ffx's theme is set to "Default"). So this is
considered fixed now. Please feel to reopen if this is not the case. Thanks for
your report!