Change Firefox color theme

[linub]

for my part,i wanted firefox to be in dark mode and apply it to supported sites.
not sure if that’s what you want, but you can check this thread and see if it helps you:

Auto dark theme on websites according to system setting

Since Firefox is a Gtk-based application, have you set a dark theme for Gtk applications? I think in KDE the option is in Settings > Application Style > Configure GNOME/Gtk Application Style. If you select a dark theme, like Breath-Dark for example, Firefox should pick up that setting and display certain websites in a dark mode. The same goes for the “internal” pages of Firefox, like about:preferences or about:config for example.