I've spent a couple hours today trying to figure out what's up with colorbox. I have a very simple page set up using your example code and some extra links to "real" products at my test store which is really a staging store for a client.
The problem ONLY shows up in Chrome (latest) and only in a new window. Colorbox does its overlay, animates a white box filling and then all of it disappears except some edge pieces. Hard to describe so here's a screen shot
Usually after that no amount of back and clicking new products gets the cart displayed. That would be a disaster with some of our doddering customers that thing their email goes to AOOL.COM.
Currently, foxycart's version of colorbox fails w3c validation -- 11 errors. I doubt css is the problem though. There is also a format error: "Resource interpreted as Script but transferred with MIME type text/plain." on the call back to cart.php with cart=get output=json. Shows error in cart.php line -1 because its AJAX magic.
I wish there was a Chrome tool to compare DOM of two windows.
Test page is http://solari.com/badbox/
EDIT - moved test page to faster server