I used to customize icons (and bazillions of other things) when I still used Windows... I had more fun changing system utility names (Windows Exploder), error messages (Your computer wants to be alone. Please do not read the end of this message, as it needs its space.), and other parts of the interface (the Start button would read 'Break' when inactive and 'FUCK!' when pressed) to horrify anyone who used the machine and thought that they had really screwed something up.
I don't use an interface with icons anymore, but I will occasionally change my mouse cursor to a small, furry animal and enable mouse trails so that I can pretend that badgers are taking over my machine.
Jeff