I'm Miles from the Social Care team at Shopify. It's great to hear you're using Printful for your print on demand store. When it comes to selecting a theme, the most important part is that it's the right theme for your brand and business - not to cater for an app. At the end of the day, Printful is the supplier of your products, and how you promote those products in your store is totally up to how you'd like to represent your business.
All apps on the Shopify App Store are designed to integrate perfectly with any theme available on the Shopify Theme Store. My suggestion is for you to play around with a couple of the themes found there (including Shopify's free themes) to find out what works best for you.
If you'd like a more personalised recommendation, can you reply to this thread with a list of features you're looking for? Think about:
Let me know by replying to this thread and I'll make some recommendations for you.