Winterboard, the king of theming apps on Cydia, has been updated to allow use on ARM64 devices such as the iPhone 5s. At this point in the game, however, be careful to choose compatible themes. Older themes will not work properly on iOS 7 devices.
I have just attempted to get Ayecon to work, I knew it wouldn't render properly but it didn't cause a Safe Mode crash. Looking forward to seeing some new themes. No doubt I'll probably buy a fair few and copy the icons from each and make my own custom theme.