I'm still using the factory default theme. My time spent in the music room is already too limited for my taste. I'd rather spend that time working on music.
I like that users are able to upload their own themes and I think its great that we have folks who are willing to put time and effort into contributing their work. But I'm just not big on the look and feel of Fisher-Price style interfaces and most of what I've seen uploaded so far tend to move the needle toward that rather than away from it.