/*
For more, see:
https://stackoverflow.com/a/6182661/1475817
*/

.outer {
    display: table;
    position: absolute;
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
}

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

.inner {
    margin-left: auto;
    margin-right: auto;
    /* Whatever width you want */
}