/usr/share/themes/ is in gtk3-engines-xfce 3.0.1-2.
This file is owned by root:root, with mode 0o755.
..
/usr/share/themes/Xfce/
/usr/share/themes/Xfce-4.0/
/usr/share/themes/Xfce-4.0/gtk-3.0/
/usr/share/themes/Xfce-4.0/gtk-3.0/gtk.css
/usr/share/themes/Xfce-4.2/
/usr/share/themes/Xfce-4.2/gtk-3.0/
/usr/share/themes/Xfce-4.2/gtk-3.0/gtk.css
/usr/share/themes/Xfce-4.4/
/usr/share/themes/Xfce-4.4/gtk-3.0/
/usr/share/themes/Xfce-4.4/gtk-3.0/gtk.css
/usr/share/themes/Xfce-4.6/
/usr/share/themes/Xfce-4.6/gtk-3.0/
/usr/share/themes/Xfce-4.6/gtk-3.0/gtk.css
/usr/share/themes/Xfce-b5/
/usr/share/themes/Xfce-b5/gtk-3.0/
/usr/share/themes/Xfce-b5/gtk-3.0/gtk.css
/usr/share/themes/Xfce-basic/
/usr/share/themes/Xfce-basic/gtk-3.0/
/usr/share/themes/Xfce-basic/gtk-3.0/gtk.css
/usr/share/themes/Xfce-cadmium/
/usr/share/themes/Xfce-cadmium/gtk-3.0/
/usr/share/themes/Xfce-cadmium/gtk-3.0/gtk.css
/usr/share/themes/Xfce-curve/
/usr/share/themes/Xfce-curve/gtk-3.0/
/usr/share/themes/Xfce-curve/gtk-3.0/gtk.css
/usr/share/themes/Xfce-dawn/
/usr/share/themes/Xfce-dawn/gtk-3.0/
/usr/share/themes/Xfce-dawn/gtk-3.0/gtk.css
/usr/share/themes/Xfce-dusk/
/usr/share/themes/Xfce-dusk/gtk-3.0/
/usr/share/themes/Xfce-dusk/gtk-3.0/gtk.css
/usr/share/themes/Xfce-kde2/
/usr/share/themes/Xfce-kde2/gtk-3.0/
/usr/share/themes/Xfce-kde2/gtk-3.0/gtk.css
/usr/share/themes/Xfce-kolors/
/usr/share/themes/Xfce-kolors/gtk-3.0/
/usr/share/themes/Xfce-kolors/gtk-3.0/gtk.css
/usr/share/themes/Xfce-light/
/usr/share/themes/Xfce-light/gtk-3.0/
/usr/share/themes/Xfce-light/gtk-3.0/gtk.css
/usr/share/themes/Xfce-orange/
/usr/share/themes/Xfce-orange/gtk-3.0/
/usr/share/themes/Xfce-orange/gtk-3.0/gtk.css
/usr/share/themes/Xfce-redmondxp/
/usr/share/themes/Xfce-redmondxp/gtk-3.0/
/usr/share/themes/Xfce-redmondxp/gtk-3.0/gtk.css
/usr/share/themes/Xfce-saltlake/
/usr/share/themes/Xfce-saltlake/gtk-3.0/
/usr/share/themes/Xfce-saltlake/gtk-3.0/gtk.css
/usr/share/themes/Xfce-smooth/
/usr/share/themes/Xfce-smooth/gtk-3.0/
/usr/share/themes/Xfce-smooth/gtk-3.0/gtk.css
/usr/share/themes/Xfce-stellar/
/usr/share/themes/Xfce-stellar/gtk-3.0/
/usr/share/themes/Xfce-stellar/gtk-3.0/gtk.css
/usr/share/themes/Xfce-winter/
/usr/share/themes/Xfce-winter/gtk-3.0/
/usr/share/themes/Xfce-winter/gtk-3.0/gtk.css
/usr/share/themes/Xfce/gtk-3.0/
/usr/share/themes/Xfce/gtk-3.0/gtk.css