Imagine how upset he'd be when he t turns that N73 on for the first time and tries to change the wallpaper on his theme? Sure, it can be done, but not without embedding it in a theme, which is exactly what Theme DIY does. And it sucks.
To the original poster, I would ask questions too if I was about to spend $400 on a device I didn't know a lot about. Next time try the search button or the s60 wiki for any other questions.
To Angel, while it is true that you newb it up from time to time, you didn't do so this time, so I apologize.