Whoa...
People need to relax around here a bit. You see - I think it is okay to have something a bit fun in the enteprrise product. It is for end users after all - if they don't want to look at it - they do not have to.
I also find it amusing that some think that Microsoft actually said "OK, let's see - OWA features... two themes or <insert the feature you are missing>? Themes it is!".
Developers are allowed to have a bit of fun too. If people understand and approve that Google lets their employees work on creative projects - is it so hard to imagine that someone at MS actually worked overtime to create those themes just because they thought it would be cool to have? Could it be??