.fullscreenDiv {
    width: 100%;
    height: auto;
    bottom: 0px;
    top: 0px;
    left: 0;
    position: absolute;
}
.center {
    position: absolute;
    width: 100px;
    height: 50px;
    top: 40%;
    left: 50%;
    margin-top: -25px;
    margin-left: -50px;
}