title: Nested resource dictionaries
body: <p>I want to define multiple themes for my application and switch them every time I like, but I want to put styles of each control of each theme in a separate <code>ResourceDictionary</code>  so it makes files in a businesslike style and I can manage them fast and easily.
  But the problem is:Styles of the nested Resource Dictionaries does not apply.
  any suggestion?
  thank you.</p>