Xfce is actually GTK3 and GTK2 hybrid???

My PureOS only has SDDM (replaced against GDM) and Xfce 4 (replaced against GNOME) installed, but has Breeze-like flavours like xfwm4 themes, Breeze-dark icons and GTK controls, QtCurve-GTK2 controls, Plasma 5 system settings (without K Plasma shell also actually present) with SDDM-KCM and GTK-KCM, Qt5CT and so-on stuff to tweak Xfce 4 looking like Plasma 5.
In SystemSettings5 when I use Breeze-dark for GTK3 and QtCurve for GTK2, Xfce4-goodies applies Breeze-dark but the base shell applies QtCurve but nowhere to set dark flavours. For fully dark themes applied to the Xfce 4 shell I need to use Breeze-dark for both GTK3 and GTK2.
There, Xfce 4 is actually GTK3 and GTK2 hybrid?