style.css

html, body {
    height: 100%;
}

.vertical-center {
    min-height: 100%;

    display: -webkit-box;
    display: -moz-box;
    display: -ms-flexbox;
    display: -webkit-flex;
    display: flex;

    -webkit-box-align: center;
    -webkit-align-items: center;
    -moz-box-align: center;
    -ms-flex-align: center;
    align-items: center;
    width: 100%;

    -webkit-box-pack: center;
    -moz-box-pack: center;
    -ms-flex-pack: center;
    -webkit-justify-content: center;
    justify-content: center;
}

.jumbotron.vertical-center {
    margin-top: 20%;
    margin-left: 15%;
    width: 70%;
    height: 30%;
    background-color: white;
}

.row.bottom-padding {
    margin-bottom: 5px;
}

.row.extra-bottom-padding {
    margin-bottom: 20px;
}

.btn.start {        
    padding-left: 15px;
    padding-right: 15px;
}