Using KDE Neon 5.17 with Firefox 69.02, if I use a dark theme with KDE (Breeze Dark), I can't override Firefox to fully be a light theme. This was working fine up until today, so something recent changed in an update. KDE is set to "Breeze Dark" as the theme. KDE's option for GKT2 and GTK3 theme ( settings -> gnome/gtk application style) are set to "Breeze" (light theme). Firefox's theme is set to "light" (it was working with "default" as well before). Almost all GUI elements are defaulting to dark. Menu items, tooltips, scrollbars, checkboxes etc. are dark. Browsing to bugzilla.mozilla.org (which seems to detect if you're in a dark theme/mode) will display in a dark css theme. I've set the following in about:config, but it doesn't seem to fix anything: ui.systemUsesDarkTheme;0 (fixes the CSS bugzilla issue but nothing else) widget.content.gtk-theme-override;Breeze (or Breeze:light) widget.content.allow-gtk-dark-theme;false Basically, despite the fact that i've flipped every switch I can think of to tell firefox "use a light color scheme", it still wants to be in the system's overall dark theme. And again, this was working fine a day ago.