/usr/share/themes/Xfce-flat/ is in gtk3-engines-xfce 3.2.0-2.
This file is owned by root:root, with mode 0o755.
..
/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