Unfortunately Shoptify is a platform that I am not too keen on. If you want a solid platform with tons of FREE themes then use WordPress, it may take a little adjusting at first but in the long run you will be much better off.
As far as the specifics to your question, good luck in your hunt, its going to be difficult to find FREE Premium Themes for that platform. WordPress is open source, Shoptify is totally for profit. May not be the answer you are looking for but I hope it helps some!
Unfortunately Shoptify is a platform that I am not too keen on. If you want a solid platform with tons of FREE themes then use WordPress, it may take a little adjusting at first but in the long run you will be much better off.
As far as the specifics to your question, good luck in your hunt, its going to be difficult to find FREE Premium Themes for that platform. WordPress is open source, Shoptify is totally for profit. May not be the answer you are looking for but I hope it helps some!