body, html {
    height: 100%;
}
  
.bg {
    /* The image used */
    background-image: url("img/atardecer.jpg");
  
    /* Full height */
    height: 100%;
  
    /* Center and scale the image nicely */
    background-position: center;
    background-repeat: no-repeat;
    background-size: cover;
}