The look & feel of system components on Windows has actively regressed starting from Win 8 and subsequent releases. No idea about the situation on Mac OS. But AIUI, both Qt and GTK+ will give you a good enough UI in practice. Perhaps someone will even split out the SerenityOS widgets (which are truly best-in-class, meshing the advantages of "classic" and "modern" UX) as a multi-platform framework, much like has been done wrt. the browser engine.