Hmmm that is an interesting question. Before I used to be so crazy and focused on themes and making my store look perfect. I realized, no need! I started using Debut and Brooklyn. One big picture banner at the top and we are good to go.
Of course after I have a lot of sales I would like to buy Parallax but that is only for personal enjoyment. I think even the free themes work well.
Also did you know Shopify gives you 1 hour free design time for their themes? They can't do everything but small tasks they will do for free, it is awesome!