Hello MTWK,
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.