html {
  background: transparent none no-repeat fixed center center / cover;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

@media only screen and (min-device-width: 550px)  {
  html {
    background-image: url("login_background.png");
  }
}