@_@ I'm using a modified version of Garland and tried to get it working with no luck. I added that snippet to style.css, but nothing changed. Did you have to code something else?
Thanks =)

Update: Sorry for not noticing before, but I was just checking results at the category site, which uses the grid module for displayig its products. Actually, I had to enter the specific product's node in order to see the changes to the 'Add to cart' button. In order to accomplish the same for the grid display, I had to edit likewise the uc_catalog.css =) Thanks.