You are using either an older version of ZenCart or an old stylesheet as it is stamped $Id: stylesheet.css 1223 2005-04-22. You issues are fully with the main design background size and the tables for the remainder that are larger than the design.

If this is an older table based cart. one you might upgrade and bite the bullet now and stay inline for the future. Or wait for another that remembers the tabled design and how to adjust it.