#mobile-nav-bg {
  display: block;
  width: 100vw;
  height: 100vh;
  background: rgba(0,0,0,.25);
  position: fixed;
  z-index: 99;
  top: 10000px;
  left: 0;
  opacity: 0;
  transition: opacity .25s ease-out;
}

body.mobile-nav-expanded #mobile-nav-bg {
  top: 0;
  opacity: 1;
}

@media only print  {

  #mobile-nav-bg {
    display: none;
  }

}