First download the Aeon skin onto your local computer from XBMC.org. It should have 4 folders and a couple of files. Go to whatever you’re using for FTP and create a new folder (or directory) named Aeon here: Applications/XBMC.app/Contents/Resources/XBMC/skin. Drag your new Aeon folders and files into that folder.
Now go to XBMC on AppleTV. It probably has Confluence of the default skin. Cycle thru the skin settings until you find APPEARANCE. Cycle thru the skin settings until you stop at Aeon.
Aeon will now become your skin.
I personally find that Transparency! is an incredible skin—very clean, responsive and not at all buggy. You can install/use Transparency! by using the same process you used for Aeon.
There are other ways to do all this including installing new skins thru the XBMC SVN Installer.