center element center in fixed css code example
Example 1: how to center a position fixed element horizontally
left: 50%;
margin-left: -400px; /* Half of the width */
Example 2: how to center with position fixed
position: fixed;
width: 500px;
height: 200px;
margin: 5% auto; /* Will not center vertically and won't work in IE6/7. */
left: 0;
right: 0;