Rename "Generic_OSX_Icon.icns" to one of the options below and put
inside of the icons folder if you want the simple look...

os_cougar.icns > OS X Mountain Lion 10.8
os_lion.icns > OS X Lion 10.7
os_snow.icns > OS X Snow Leopard 10.6
os_leo.icns > OS X Leopard 10.5
os_tiger.icns > OS X Tiger 10.4