Help with Page Width in Pav Wine Theme

Hi
I am transferring my shop from Zen Cart to Open Cart and would like to use the PavWine theme.

All good so far but I can not work out how to make the whole layout wider in the screen.

Because it is so narrow with so much background all my top categories as squashed up

could someone please tell me what file contains the widths I need to change please

here is the shop url so you can see what I mean. I would ideally like the width to be about 75% of the screen.

http://www.horsemansshop.com

Thanks if someone can help me .
Johanne