Remember, Winterboard is the tool that allows themes to be downloaded and run. It is not designed to be a theme, it enables them. You have to find a theme you like on Cydia (that will run on your device and iOS) and download it. Then, you use Winterboard to activate it.
The ThemeStore is different, but I should imagine it's pretty empty because themes have not been updated for iOS 7. Give 'em time....
Hope that clarifies.