You can select a suitable theme for your store from the large collection of themes available as per your subscription plan.
Here's how you can Select themes for your store.
From the left panel in your dashboard select Online store -> Products -> Store -> Select Themes
If you are on a Free Plan you will be able to select only Free Themes. In order to apply Premium themes to your store you need to Upgrade your Free Plan to a Premium Plan.
Here is a Sample Theme