pages disappear then changing menu style

I am using Atahualpa 3.6.1 and WP 3.1

When I replace the code to change the style of the menu (using code in the 'Styles' directory), the entire content (execpt for the page background) disappears. When I either replace the included code or remove all code (in the 'Add HTML/CSS' section), the site appears again.

I must be doing something wrong, but I have no idea what. Anyone have any ideas?

Thanks for your help.

Jon

Site affected is http://antoniositalian.com/ai/