Theming your jailbroken iOS device is certainly possible, but it's never been remarkably straightforward or simple. Theme it is a new (and free) Cydia app that brings easy theming to jailbroken iOS devices.
Theme it is reminiscent of the iTunes App Store in a lot of ways, including its looks, except you only use it to download and apply themes. Some themes are free and some are paid, but the app itself is free so you don't have to pay a dime if you don't want to. Themes go beyond just changing your dock and your homescreen icons, but go so far as changing interface elements as well. If you're curious what themes look like, just check out the currently available themes. Each comes with interface screenshots and gives you compatibility statistics so you don't buy and/or download a theme you can't use with your specific device.
You can download Theme it from Cydia or form the Theme it site and install it via SSH. It requires a jailbroken iOS device to run, but once you've jailbroken and installed it you can customise to your heart's content.