Cydia Theme Center actually divides the customize themes/icons into two categories of HD (iPhone 4, Retina display), and SD (iPhone 2G, 3G, 3GS).
All the paid and free themes are mixed at Cydia Theme Center but at the moment more free SD themes are there as compare to HD themes.
Hopefully, Cydia Theme Center will grow by the passage of time. So, visit Cydia Theme Center right now
just by navigating to Cydia on your iPhone and it will appear on the main page, where it also provides some information about upcoming Cydia 1.1.
Have a look at Theme Center and write us your comments about it.