your value for top and margin-top are not correct, cause it is based on center and your boarder is down 65%. try this instead:
.center_box {
background: #000;
position: absolute;
top: 65%;/* the tune you need to start with */
left: 50%;
margin-top: -235px;
margin-left: -200px;
width: 400px;
height: 235px;
}
You could even use a linear-gradient on body if you want to include only young browsers : http://codepen.io/anon/pen/EImiz