The only way i believe it can be done ATM is by playing around with terminal.
Its not like the dock or system icons, where they are very customisable.
I know i have read in a forum here that someone has done it.
Im just having a look around, and ill post the thread that it is in as a EDIT.
It was a certain member so if you want might be worth giving him a PM as he might not read this thread ...
Ill be back
Cheers
EDIT : found it and its here @
Post your Dock Screenshots. The particular member is
DarkestRitual. Read what he has to say as i didnt go through the whole thing again. Try back a page too.
Hope it helps a little.