The new Yahoo! Messenger beta is working well for me, and the fact that it supports themes is an interesting development.

At this time, I haven't found any useful documentation online on how to create new themes for the Mac client, but I've modified the "Default" theme to have some darker elements, calling it "TheDark".

Once it is complete, I'll post my Yahoo! Messenger theme "TheDark" here for free download, but for now, here is a preview: