Out of the Sandbox Purchases
After the payment is complete, there will be a download link which you can follow to download the theme's ZIP file. An email will also be sent containing this download link. After the theme has been downloaded, log in to your Shopify Admin and go to the Themes > Manage Themes section. Upload the entire ZIP file and then publish it as your 'Main' theme (note that it will also be used on mobile devices). From the Theme Settings you can select your desired 'style' from the left column, then save your settings before customizing further.