What is the exact css code to put a background behind a centered div tag?

[pzqwpo]

I want to center my website and behind the centered div tag i want to have a plain gray background but i can't seem to get any codes to work.

Please give me the exact codes and where to put them...i dont know if i should put it in a css or right in the page code.

 

[MK3Design]

#div-id {
        width: 100%;
	margin: 0 auto;
	background: url('imagehere.jpg');
	background-repeat: x;
}