Picking a new theme is a great opportunity for everyone to re-fresh their online appearance. We are encouraging all stores to review and take advantage of new "responsive" themes that are designed to display well on mobile devices. Please take a look at our themes list for responsive and mobile friendly themes.
We particularly encourage stores with customized themes to consider "off the shelf" alternatives. If one such theme is discovered to be acceptable, a store will not only avoid costs associated with upgrading their current theme, they will likely avoid costs in future upgrades.
D7 Taxonomy:
Developer FAQ [1]