Creating the MessageBox child window user control:
At first we will create our own user control. Then we will use it in a sample application. Start Expression Blend and choose Control library project template (in order to generate a DLL so that it can be added to in any Silverlight application), then remove the MainPage.xaml from your solution and add a new Child Window.
In expression blend, now you can easily change the appearance of this child window. You can change the information bar back ground, change the whole layout, the power is on your hand. Removed all the default assets and included the followings in the layout root of the child window: