.div-center {
width: 400px;
height: 400px;
background-color: #fff;
position: absolute;
left: 0;
right: 0;
top: 0;
bottom: 0;
margin: auto;
max-width: 100%;
max-height: 100%;
overflow: auto;
padding: 1em 2em;
display: table;
}

div.content {
display: table-cell;
vertical-align: middle;
}