Hi!
As aMule is using wxGTK you can simply change your gtk theme and aMule will have a new look. Change your gtk theme (gtk1, not 2 - you should have something like ~/.gtkrc) and restart aMule. It's really that simple.
I forgot to mention that this change will also affect any other gtk based app you may use, so think of it and don't worry if more than aMule changes after you switch your theme.