The toolkit decides how their applications look based on whatever theming engines they use. The desktop doesn’t have control over that. So GTK applications use GTK themes, libadwaita applications use libadwaita themes, libcosmic applications will use libcosmic themes. We’ll try to automatically generate similar-ish GTK/KDE/etc. themes if possible, but it’ll depend by what specs those toollkits support in their theming.