To anyone who doesn't get it yet, I just figured out how to do this, here's how: Use this older version of ThemePark (works on Tiger and Leopard) to modify the Extras file using resources from Yosemite. Then save that file to somewhere like your desktop. Now make a backup copy of /System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/Resources/Extras.rsrc and then delete the original. Drag in your modified file and restart.
Screenshot that it works?