Changing Thumbnail background

I am having issues finding the options in order to change the background of a thumbnail image in the grid of a product gallery within the standard template. my design has a black background and the white boarder it is adding is extremely unappealing. Maybe I’m just blind… but I can’t seem to find the tab that lets me change this to either transparent or at the very least black.

This is what I’m talking about: http://bloodboundbooks.net/bloodboundbooks/products_categories/novels_novellas/

Also, is there a way to extend the layout beyond 2 rows, I found the option to add more spacing between the 2 rows but nothing about extending the number of rows.

Your help is appreciated.