There are certain situations where you will need to access resource dictionaries outside of your application. For example, you may want to share one resource dictionary among several applications that share the same styles.
What you want to do is use WPF’s pack URI Scheme.
Remember to include a reference of the assembly containing the resource dictionary (YourAssembly) in the application project. Then, all you have to do is use this rather strange looking syntax to access that resource dictionary within your xaml.
There are many variations of this syntax that you can use. Which one is right for you depends on exactly what you are trying to do. The above syntax should work for a majority of the situations. Accessing these resources can be done in both xaml and the code behind.
For more information on the pack URI Scheme syntax, follow this link:
Note - this syntax is not only for resource dictionaries. This is useful for accessing several types of data files like images for example. This is commonly used for accessing resource dictionaries, but it is not limited to that.