I have the same problem. I want to modify the buttons template from the main template of the rad message box.
All I want to do is to keep the white color of the foreground and BorderBrush, even if the phone's theme is dark or light.
I tried to create 2 buttons with styles inside this stackPanel, but it doesn't replace the existing buttons:
<StackPanel x:Name="PART_ButtonsPanel" Orientation="Horizontal"></StackPanel>
The question is: How and what should I style? This stackPanel or something else?