One of the most common topics in the jailbreak forums is always theme editing and customization. To really be able to truly customize a theme, you need to know how to SSH into your iPhone. Anyone who is already familiar with web development and using an FTP client should be right at home. For the most part, FTP clients are also capable of SSH. If you don't have a client, don't worry, there's plenty of free ones.
And now for the typical disclaimer - if you aren't comfortable messing with your phone's file system and the possibility of having to completely restore if something goes wrong, don't proceed any further. As always, we aren't responsible for any damage you may do to your phone, so proceed at your own risk. If this all sounds okay to you, read on iPhone ninja!