@media (min-width: 1281px) {
  html {
    background: url(../img/fondo/mantenimiento.png) no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
  }
}

@media (min-width: 768px) and (max-width: 1281px) {
  html {
    background: url(../img/fondo/mantenimiento-mobile.png) no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
  }
}