Windows 8, the upcoming OS from Microsoft has many changes and one of the them is the new Windows Explorer. The Windows Explorer in Windows 8 sports the ribbon UI found in office and Windows Live suites. The ribbon UI gives easy access to options and features which were not directly accessible in Windows 7 and other versions. If you want to customize the icons available on Windows explorer, then you can try out the freeware tool Ribbon Icons Customizer.
The free app can change the icons available in the ribbon UI of Windows explorer and also reset them back to original.
Once you have downloaded the app, first run the Resources.reg registry fix and add its contents to your Windows Registry. The .reg fix basically only takes ownership of some registry key/s and does not add any new one.
Once you have done this, just run the portable app and select the icon you want to change and replace it with the icon file. There are many toolbar icons set available free of cost and you can download one of them and customize the explorer icons.
You can always restore it back to the original set using the restore option. Here is a sample Windows explorer, after the icons were changed (img credit: TheWindowsClub).