In my symbian life I've always appreciated pizero's themes, so hey! Thanks pizero for your creativity.
Regarding your questions, well, the previous maemo devices could change several elements of the ui, including icons and backgrounds, and you'll find several threads in this forum. Look for themes like plankton, lcars, titan, velvet (to name some I have on my device).
Since the os is open I don't think there will be problem to make installable packages - in the standard .deb format of debian.