Hiya,
Instead of stylesheet.css, you should look in index.php.
First table in body controls the dimension of your site.
This is what you can do.
A. Find Table Tag,
B. Make border="2" instead of default value of "0"
C. Set a BGCOLOR="RED" to check which table you have modified.
D. If you got the right table - time to change it's width in % (rather then pixels)
Roll back other changes, once you get right width for your cart, ie, border should become 0 and bgcolor should be deleted.
Hope this helps,
Cheers,