I used your sample CustomHeaderedContentControl to display a two message box and it appears as the background was transparent and the Main Title and Message were black and hardly visible. How to fix this? See image.
P.S. Even just using RadMessageBox.Show() without the custom content control exhibits the same issue, in your sample project it works OK I presume because you are using ViewModel to control theme. Regardless it should work out of the box correctly in the default dark theme. I have some slightly different code to account for dark and light theme, but I see no way to control text forecolor as a propertie of the default RadMessageBox (without custom content control). Trying to edit the content control to set fore color had no affect.
OK Nevermind, I solved it. in App.xaml.cs the help engine I was using changed the rootrame to:
RootFrame = new B4.AppLibrary.CustomTransitionFrame();
I changed it back to = new PhoneApplicationFrame() and that fixed it.
Sorry for the False alarm. Great Product. :)