Creating an AmiKit theme consisting of the startup images, wallpapers, window borders and launcher graphics is not so simple because at least the startup and wallpaper images have to be provided in several screen modes.
Anyway, I'll post the guide on how to create such an AmiKit theme later.
Btw. Magellan supports themes as well. They are different themes, though, which consist of wallpaper, sounds, settings, etc. You can Load/Save themes using the right-mouse menu system of Magellan environment. Themes are available from eg. Aminet, click here