I'd add a # before DDDDDD, but only because some browsers are fussy about that.
Put simply, your CSS is working fine. You don't have any text in your document, so there's nothing to align. If there were text somewhere, it'd be centered. Try adding to text and you'll see.
I think what you want to do is set the margins to that white div to auto, like so:
#whitecontent {
margin-left: auto; margin-right: auto;
}
That will set that div to the center of the layout.