All the OS icon files, menubar, buttons, windows, scrollbars are contained in an encoded data file which you can't open without a specialized application called ThemePark.
ThemePark - Geekspiff
The big problem right now is that ThemePark was written for 10.5. While it was released for 10.6 it is very unstable (You can only use the arrow keys to navigate or it crashes) and a lot of features are missing.
The other problem is that because the files are encoded some of the contents aren't discovered/decodable yet.
Once you have ThemePark, you can open the edit the ArtFile, SArtFile and Extras data files that specify the OS's appearance.
Editing iTunes 10
You can then use a application like GIMP of Photoshop to edit/create your own icons and replace the old ones.
The way this is done is by making a new icon, or copying the old style. Editing it, and saving it as a .png and then using ThemePark to change the icon by dragging the new icon onto the old one.
Applications icons are always stored within the application so editing those is easier and doesn't require special software to see the icons.
To see the icons of apps, right click on the application and choose show "Package Contents"
The icons are almost always stored in the "Resources" folder. The applications dock icon ends in .icns and usually is called the same as the application.
To edit these you copy the .icns file to the desktop and open it with Preview. Then you save it as a .png file and open it in your image editor.
Then you need to save it as a .icns file again, this can be done with Apple's "Icon Composer" (Included in the Developer Tools)
All in all, a very laborious and time consuming undertaking.