Together with the "nifty-default-controls" project - which provides the Standard Controls - you can already start to use the shiny new awesome grey design!
A released version will be available together with the upcoming Nifty 1.2 Release.
But remember, Nifty is not at all about the controls only. You can build whatever GUI you like! This new style is meant as another demonstration how shiny and nifty you can make GUIs look with Nifty GUI. But when you are happy with the look of the new style you are of course free to use it or modify it or just stare at it, like I do ;)