Install ShapeShifter

ShapeShifter is a revolutionary new product that lets you change the overall appearance of your Mac using 'themes'.

We're not talking about just desktop backgrounds and icons here, we're talking about everything - the look of windows, menus, apps, buttons, absolutely everything. You don't wear the same clothes every day, your house doesn't look exactly like your neighbor's - why should the computing interface you use every day be any different.