To center an element you need three things:
- a valid DocType
- an element with a width
- that elements right/left margins set to auto
You have some other issues to consider first though.
- Your footer, #colophon, is 100% wide so it cannot be centered.
- Inside #colophon, you have #supplementary which is 100% wide PLUS 250px (your 125px left/right padding).
- In #supplementary, you have #'s first, second and third which you probably want to look like 3 columns.
Your best bet would be to figure out how wide #supplementary is and center that element.
For more information on how the padding in that element is breaking your layout, see the box model here
. The box model says that whatever you put inside an element cannot be larger than that element. margin/padding/border all count when figuring width/height.