Jemsen Harris addresses the mystery of the third theme for the new Office UI. A few posts have shown or referenced a silver theme in addition to the blue (Luna) and dark gray (Obsidian) themes in the beta 2 release. Turns out the third theme is something we'll not see first-hand until the next public release - if there si one before official.
I'm looking forward to seeing how this looks in actual use (as opposed to the screen shot). The Luna UI is very pale and lacks contrast. The Obsidian theme is better but looks awful in Outlook's main views because the toolbars are a light gray and the rest of the UI is dark. That color scheme may work on a Microsoft keyboard or mouse but it's hard to look at all day long on the screen IMO.