Sub-Theme

After days of trial and error in customizing CSS for my store-front, I'm wondering if there is a possibility to somehow create a theme just for the store itself using one of the WPMU provided themes like GridMarket. If this is possible so that I can keep the premium Karma Theme for every other portion of my website and a specific theme just for my storefront, that would be great. Still being somewhat new to WordPress customization, I would appreciate any assistance here as I don't want to mess up any of my website's theme.