Most people who have used OS X know about the ability to change icons by right-clicking on the program and selecting Get Info.
While this does technically change the icon, it would take forever to do that for each icon, and you would inevitably miss some of the icons, only to see the garish difference later. And, as luck would have it, it would be in the middle of a presentation or something of the like!
Say hello to CandyBar! This awesome little app runs officially on any OS X version from 10.2 on up through 10.8, and I believe it works beyond 10.8, though I’m not sure. While the app isn’t technically supported, it still works fine! I used it on my iBook G4 and it worked great, other than a minor problem that I quickly resolved. You can read the developing company’s official blog post on the support here.
You can download the newest version from the above link (which provides a Serial Code) or you can get it from MacUpdate at https://www.macupdate.com/app/mac/8948/candybar, which also supplies the same Serial Code. The version that supposedly works for 10.2 and I know works for PowerPC 10.4.11, Candy Bar 2.5.1, can be downloaded here: http://iconfactory.com/software/candybar. However, this version doesn’t have the free code available, though the trial/freeware version works the same as the full version (as far as I can tell.)
As for how to use it, it’s really simple! You start up CandyBar and it loads in the icons of pretty much everything on the system. If an app is in the Applications folder it will show up in the “Applications” tab. The Applications tab will take a little longer to load than the default icons tab, as it has to work through all of the apps, but once it comes up you just drag the icon you want into the well of the app you want to change (in png/ico/icns format, I’m pretty sure all of them should work.) Once you’ve swapped out the icons you want select either Apply System Icons or Apply Application Icons button, depending on which tab you’re in.
You can also customize the volume images as well as customizing icons in an app. Those two functions behave basically the same, except that with the application customization you have to drag the app into the program.
The minor problem I said I ran into was that certain icons wouldn’t show up correctly when applied through CandyBar. You can check out this question on the Apple Stack Exchange site that was what I followed to get it working (and yes, for those of you who astutely recognized the stack exchange flair on the side of the blog, the asker of the question was in fact, yours truly, RPi Awesomeness. That’s how I got this working originally.)
One caveat on the answer. The program (on the question, not Candybar) only allows you to load up to 250 icons at a time. Beyond that you would have to wipe your library (inside the app, you can export and they will be fine) and start anew.
All in all, this is a great app for those who want to customize their system without having to get all nitty-gritty inside the system files on their Mac.