Where to Begin to Design a Theme

Hi all,

I'm new to these parts, and this seems to be the best place, so far,  
to ask a question like this. I really like the potential of GNU/Linux  
to allow for complete control by me of my computer interface. Being a  
designer, as well as a semi-programmer, a major part to control would  
be the desktop and various applications.

So, I guess what I'm trying to say is, how do I create a theme for  
GNOME, and what is the potential to create? Can I completely mod the  
whole system? Are there conventions I'm stuck with? Or do the tools  
allow for omnipotent control?

I'm excited by this strange new world, but I have a lot to learn.  
Hopefully you all can teach me.

-Zach