add
margin-left: -200px; (half of the div width) and
left: 50%; to the div properties. That way, you're making it 50% from the left of the page to the middle of the div.
EDIT: You'll probably wanna use this too to make sure it acts properly
...
code:
div {
-moz-box-sizing: border-box;
box-sizing: border-box;
}
/ 
