Replying to How customizable can a mac be?

i remember when i used to have a PC, you could pretty much change anything  on it. i was wondering how much you can do with macs...

the bars that the computer has on top of most windows (finder, ect...) are sort of bland. how much can i change em up? what are some other cool things you can customize?

i know stuff like the pictures in the finder and the highlighting color, but thats it. im looking for something more though