I'm really not sure how a theme can be in beta, but this one sure is. I was looking around at the themes, and I couldn't find one that I really liked. A lot of reading and about 6 hours later, this is what I've came up with. This is the very first theme I have ever made, and I have only tested it on my current window setup.
One question though for any other theme creators out there. Is there a way to customize the highlight color of the menus (File, Edit, etc.) on mouseover? All my menus appear to have a blue highlight (except for the items that are disabled).
If anyone has any problems with other window configurations, I'll do my best to try and fix it.
Screenshot:

Please excuse my poor organization. I'm working on that project now that I have a better looking interface.
Download Link:
http://www.mediafire.com/?vna3hxa21tfq8oaTo install:
1. Download the file and copy it to C:\Program Files (x86)\MusicBee\Skins\ (Any subfolder will work, or just the root)
2. In MusicBee go to View>Skins>(Path) Path will be the location to the theme that you saved in step 1.