Is this possible with css (I can probably manage it with javascript):
Its basically a margin: auto effect, but i want a different background for each margin, so i’m looking for divs with auto-widths on either side of the (fixed-width) center div if that makes sense. Probs better visualized:

http://jsfiddle.net/guc9V/1/
Bit of a hack but it works.
I didn’t use your width value as jsFiddle would need resizing to see how it works.
This essentially tells the divs to act as table-cells, so they will fill the containing div, which acts as the table. If you didn’t specify the width of the middle div, each column would be 33% wide.