body {
    background: url('https://3.bp.blogspot.com/-uRWghE6vNG0/Wxqe5cGyPxI/AAAAAAAABqo/4LuDw6t8GPYRkwEh1qMcQXGYJOzr-Kz2ACLcBGAs/s1600/gunung-bromo.jpg') no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    background-size: cover;
    -o-background-size: cover;
}

.card {
    opacity: 0.9;
}

#nav {
    /* sticky memungkinkan elemen nav selalu diatas elemen lain*/
    position: sticky;
    top: 0;
    z-index: 1001;
}