They don’t. They rely entirely on donations (and sponsorship donations). It also mean, they have less resources to maintain and develop their software, ESPECIALLY Conqueror since it’s not as much well-maintained compared to other parts of the KDE software suite. Plus, Firefox do maintain their own web-engine, while KDE just use the WebKit one, so even more reasons that Firefox can’t substain with the resources KDE currently has.
Maybe you would be interested into maintaining Gradience instead of making a new LibAdwaita theming app from scratch ? Just thinking, haven’t tested it but I don’t see any straight reasons for one over the other outside the maintainership. https://github.com/GradienceTeam/Gradience