/usr/share/themes/Xfce/ is in gtk3-engines-xfce 3.2.0-3.
This file is owned by root:root, with mode 0o755.
..
/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/gtk-3.0/
/usr/share/themes/Xfce-4.2/gtk-3.0/gtk.css
/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/gtk-3.0/
/usr/share/themes/Xfce-4.6/gtk-3.0/gtk.css
/usr/share/themes/Xfce-b5/gtk-3.0/
/usr/share/themes/Xfce-b5/gtk-3.0/gtk.css
/usr/share/themes/Xfce-basic/gtk-3.0/
/usr/share/themes/Xfce-basic/gtk-3.0/gtk.css
/usr/share/themes/Xfce-cadmium/gtk-3.0/
/usr/share/themes/Xfce-cadmium/gtk-3.0/gtk.css
/usr/share/themes/Xfce-curve/gtk-3.0/
/usr/share/themes/Xfce-curve/gtk-3.0/gtk.css
/usr/share/themes/Xfce-dawn/gtk-3.0/
/usr/share/themes/Xfce-dawn/gtk-3.0/gtk.css
/usr/share/themes/Xfce-dusk/gtk-3.0/
/usr/share/themes/Xfce-dusk/gtk-3.0/gtk.css
/usr/share/themes/Xfce-flat/gtk-3.0/
/usr/share/themes/Xfce-flat/gtk-3.0/apps/
/usr/share/themes/Xfce-flat/gtk-3.0/apps/gnome-applications.css
/usr/share/themes/Xfce-flat/gtk-3.0/apps/granite-widgets.css
/usr/share/themes/Xfce-flat/gtk-3.0/apps/lightdm-gtk-greeter.css
/usr/share/themes/Xfce-flat/gtk-3.0/apps/lightdm-unity-greeter.css
/usr/share/themes/Xfce-flat/gtk-3.0/apps/unity.css
/usr/share/themes/Xfce-flat/gtk-3.0/apps/xfce.css
/usr/share/themes/Xfce-flat/gtk-3.0/gtk-widgets.css
/usr/share/themes/Xfce-flat/gtk-3.0/gtk.css
/usr/share/themes/Xfce-flat/gtk-3.0/settings.ini
/usr/share/themes/Xfce-flat/gtk-3.0/xfce.css
/usr/share/themes/Xfce-kde2/gtk-3.0/
/usr/share/themes/Xfce-kde2/gtk-3.0/gtk.css
/usr/share/themes/Xfce-kolors/gtk-3.0/
/usr/share/themes/Xfce-kolors/gtk-3.0/gtk.css
/usr/share/themes/Xfce-light/gtk-3.0/
/usr/share/themes/Xfce-light/gtk-3.0/gtk.css
/usr/share/themes/Xfce-orange/gtk-3.0/
/usr/share/themes/Xfce-orange/gtk-3.0/gtk.css
/usr/share/themes/Xfce-redmondxp/gtk-3.0/
/usr/share/themes/Xfce-redmondxp/gtk-3.0/gtk.css
/usr/share/themes/Xfce-saltlake/gtk-3.0/
/usr/share/themes/Xfce-saltlake/gtk-3.0/gtk.css
/usr/share/themes/Xfce-smooth/gtk-3.0/
/usr/share/themes/Xfce-smooth/gtk-3.0/gtk.css
/usr/share/themes/Xfce-stellar/gtk-3.0/
/usr/share/themes/Xfce-stellar/gtk-3.0/gtk.css
/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