In Visio 2007, we introduced the Themes feature to make it easy to apply a professionally designed look to a diagram. In Visio 2010, the Themes feature gets a whole new user interface that takes advantage of the Office Fluent UI. The theme choices are displayed in a gallery on the Design tab in the ribbon.
Link – http://blogs.msdn.com/visio/archive/2009/09/15/themes-and-live-preview-in-visio-2010.aspx
