You need a file manager like iFile installed on your device, or use an SSH client from your computer. I think the themes are located in /Library/Themes on the device. Find the e7.theme folder, and then open Bundles. You'll find the application folders in there, and you can proceed to rename the icons. Most of the Apple apps use Icon@2x.png as their icon name, so if you don't like the one currently being used, rename it to something like Icon@2x.png.alternate and then rename the one you want to use to Icon@2x.png. Respring and you should see the changes.
I tried the above, no joy. Even went so far as to remove every 'offending' png in the com.apple.mobilesafari folder and include only the 'preferred' pngs with appropriate renaming. Tried both respringing and rebooting but the 'offending' icon shows up. Any idea what I'm doing wrong?