You can force the GTK dark theme on Firefox by search in configuration panel for allow-gtk-dark-theme and if it is false setting it to true. After a restart of your browser you should be able to get the dark theme.
- First open your firefox and type about:config in URL bar.
- Ignore the warning that pop up by clicking on “I accept the risk!”
- Search for allow-gtk-dark-theme and if both values are false toggle to true.
Tip: Right click or double click on a specific row, allow you to toggle value from true>false and false>true.
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