I think you will be pleasantly frustrated at how different OSX is from windows. a big thing I see being a problem is not having much freedom with the menubar
is there as much freedom with TinkerTool than as with silk? and I am pretty sure there are some conflicts with candybar and shapeshifter, considering shapeshifter as an option for changing icons.
also I have a few items in the menubar and occasionally when switching themes the icons won't theme. I have to login then out but was too lazy to ever try to fix it
depending on the app you have even more control as to how much you can customize it. for example if you control click an app and select "show contents" you'll see a resources folder. in that folder you'll find all the graphics and icons that make up an app. now this doens't work on all mac apps, especially apple apps as they do some weird stuff when building thier apps. an excellent example of this is the VLC Makeover Kit: http://macrabbit.com/goodies/
themepark, shapeshifter, pixadex, candybar, yahoo widget engine, silk, tweaking ressources of some apps...
but most work is done in themepark. a guikit can be really complex with all it's app skins. one has to learn to handle themepark to skin a mac properly. ;) ...or just download a complete guikit somewhere. :D
I'm tellin' you guys, TinkerTool > Silk. Silk changes all the fonts at once as far as I know. TinkerTool lets you specify what font you want where - titlebar, system font, etc. Plus it's free.