Context-click (control-click, right-click) on the desktop background?
I know that's what your looking for. It's easy to change any icon, but using a program like
CandyBar will make the changes quick and easy, and allow you to change things like the system bezels with out having to use the terminal.
Xounds is a little haxie (not in love with the term) that allows you to change your acoustic environment. I like it, and will submit my subtle ambient sound set at some point.
Unsanity has other haxies to alter your system.
There are other ways to alter the look of your os, from Doc alternatives to haxies, to clever little scripts that do wonderful things.
What are you looking for exsactly? Shapeshifter will likely become UB soon.
A little warning, often these extensions/hacks/apps don't function well (if at all) after a minor or major upgrade, or may cause other apps to crash.
Scientia est potentia. Caveat user!