mdbootstrap4-pro/html/tutorials/bootstrap/tutorial-2/2/css/style.css

MDBootstrap 4 PRO

/* YOUR CUSTOM STYLES */

html,
body,
header,
.view {
    height: 100%;
}

.view {
    background: url("http://mdbootstrap.com/img/Photos/Horizontal/Work/full page/img%20(2).jpg")no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}

.flex-center {
    color: #fff;
}