@thelinuxcast Maybe they do it similiar to how dracula does their theme or EliverLara does the nord theme? I have mine themed with nord.
I believe its a mix of gtk css themes and layouts with html.
I once tried to meddle with them to try to configure them further but unfortunately i'm not that good with css / html.
https://github.com/dracula/firefox
https://github.com/EliverLara/firefox-nordic-theme