html,
body,
header,
#intro {
    height: 100%;
}

#intro {
    background: url("https://mdbootstrap.com/img/Photos/Horizontal/Nature/full%20page/img%20%283%29.jpg")no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}