Application Global Resources In XAML
Nathan Dunlap points out that resources can be defined at the application level in XAML. This enables the resources to be used across all “pages” in an Avalon application. He then makes the analogy between this technique and using a base CSS file for your web application. Finally, he wraps up by mentioning the BasedOn property of the Style class. This property allows you to create a new style by deriving from an existing style and overriding or extending the base style definition.
The great thing about styles in Avalon is that they're much more than just decorating an element. You can completely re-define the internal composition of a visual. You can also define a set of visual triggers that alter the visual based on dynamic property notification. The more time I spend studying styles in Avalon, the more impressed I am with the implementation.