(define (my-polkit-settings config)
  (list
   (plain-file "50-my-power.rules"
               "polkit.addRule(function(action, subject) {
                 if (action.id == \"org.freedesktop.login1.suspend\" ||
                     action.id == \"org.freedesktop.login1.hibernate\" ||
                     action.id == \"org.freedesktop.login1.reboot\" ||
                     action.id == \"org.freedesktop.login1.power-off\" ||
                     action.id == \"org.freedesktop.policykit.exec\") {
                   if (subject.local && subject.active) {
                     return polkit.Result.YES;
                   }
                 }
                 return polkit.Result.NOT_HANDLED;
               });
")))

(services
  (append
    (list
      ...
      (service xfce-desktop-service-type)
      (service-extension polkit-service-type my-polkit-settings))
    (modify-services %desktop-services
      (delete gdm-service-type))))