There are a number of ways you can customize the Opie GUI:
You can select which applets appear in the task bar using the "Launcher" menu and picking "Taskbar".
You can change properties like the style, font and colors using the "Appearance" app.
You can change the Opie background, for either all tabs or on a per-tab basis, using the "Launcher" app, selecting Tabs and picking which tab or tabs should get a new wallpaper. (It's been suggested that you choose a wallpaper that has already been sized for the screen at 240x320.)
Finally, you can also use Pocket PC themes as backgrounds.