It is not mandatory to provide a string type key name to a resource binding. You can also provide a static object to the binding. In this example, we used the static value SystemColors.DesktopBrushKey in combination with the {x:Static} markup extension:
Fill="{DynamicResource {x:Static SystemColors.DesktopBrushKey}}"
As we have learned about the dynamic resource binding in the previous recipes, this example also follows the same method, and thus you can see the selected color is automatically applied to the rectangle.
There are many static keys under the SystemColors class, which you can reference in your design. This is often useful when you want your application to be in sync with the operating system's theme.