Are you referring to the theme of the tabs and menus, or of the web pages themselves?
By default, the tabs and menus should just inherit your GTK3 theme. Assuming that you are using MATE, you might only be modifying your GTK2 theme if you are using the "Appearance" menu under mate-control-center. You can set the GTK3 theme using gnome-tweak-tool or by editing ~/.config/gtk-3.0/settings.ini (see here[1]) If you want Abrowser tabs and menus to have a different theme from the rest of your desktop, or if you want to actually change the look of web pages, then I think you'll need to install themes and/or addons. Refreshingly, Mozilla requires addon developers to specify a license when uploading their addon to addons.mozilla.org, so it is very easy to check whether such addons are free. For example, this addon[2], which allows you to modify the look of individual web pages, is GPLv3. Unfortunately, it looks like they don't require users to specify a license when they upload a theme, so if you care about free culture you might need to dig for the license of a theme that you use. [1] https://wiki.archlinux.org/index.php/GTK+#Basic_theme_configuration [2] https://addons.mozilla.org/en-US/firefox/addon/stylish/
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