Hi,
First of all the correct URL for the CoolMart template's DEMO is:
www.flexiblewebdesign.com/virtuemart-2-template-coolmart/index.php
And yes, you can make any modification you would like to category browse page.
the CSS file you need to make modification is this:
/templates/YOUR_TEMPLATE/html/com_virturmart/assets/css/flexibleVM2Global.css
Learn how to use FireBug if you are newbie to CSS modification. Firebug will help you to make CSS changes in the category page.
More info about FireBug can be found here:
www.flexiblewebdesign.com/forum-virtuemart-2-template-puremart-support/342-using-firebug-for-customization-or-troubleshooting
Regards.