There are after-market launchers that include skinning ability. For her, that might be enough.
If once she gets it and desires more of the OS is themed, then the only option will be rooting the phone. Most of the visible OS cannot be themed unless the OS has been rooted (think Linux.. apps can have root access if you permit them). A great theming utility is MetaMorph, but that requires Root.
But launchers/Home replacements (which consist of the home screens and the app-slider screen), offer a lot of skinning capability for the immediately visible portions. Also, a simple background can go a long way for some people, so a nice pink or light-colored background image could do the trick as well.
One Home-replacement that is easy to use (like the default Home/Launcher) that I'd suggest is GDE. I had a lot of success with that one.