[Feature] Resource dictionaries [Differences from 3.5 SP1] To improve theme-level resource dictionaries and prevent them from changing, freezable resources that are defined in a resource dictionary and merged into a theme-level dictionary are now always marked as frozen and are immutable. This is the expected behavior for freezable resources. [Recommended changes] 
Applications that modify a resource that is defined in a theme-level merged dictionary should clone the resource and modify the cloned copy. Alternatively, the resource can be marked x:Shared="false" so that the ResourceDictionary creates a new copy every time the resource is queried.