SerenityOS is the most well known but it's a fully custom operating system of its own. For Linux you can install the chicago95 theme (includes a widget set for GTK+3) and the b00merang GTK+4 theme (doesn't help with excess padding unfortunately, but it still has proper high-contrast 3D for the widgets and color for the headerbar. The mobile-friendly responsive UX of new GTK+4 apps actually works great with the traditional 3D look.)