.container{
    position: absolute;
    top:0;
    bottom:0;
    left:0;
    right:0;
    background: rgba(0,0,0,1);
}
.inner{
    position: absolute;
    left:0;
    right:0;
    top:0;
    bottom:0;
    width:500px;
    height:500px;
    margin:auto;
}