aMule has a new skin format in version 2.2.0 and in current developmental versions.
Skins are now a zip file which you just have to put in your skin folder (see
the wiki for details) and then you can select the skin in Preferences->Gui Tweaks.
There's a download and preview site for some skins at
http://forum.amule.org/index.php?board=77.0P.S. More skins are always welcome!
If you are on a non-Linux platform please test this feature and tell me if it's working as it should.