Choosing an e-commerce theme

This is my first time setting up a website and I want to create an online shop to sell my girlfriends homemade art and crafts. Do I have to choose an existing shop theme? I don’t want to have to pay any money and all the free ones are pretty nasty. I really like the look of the clean, minimalist photographic and portfolio themes, is it possible to customise one of them with a shopping cart widget etc and make it into a shop?

Thanks