MessageBox: Set the Cancel button IsCancel to true. Now escape will close the messagebox as expected.

ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/MessageBox/Themes/Generic.xaml

@@ -259,7 +259,7 @@
                                             <Button Grid.Column="0" x:Name="PART_YesButton" MinWidth="65" Margin="6,0,0,0" Visibility="Collapsed" Content="{TemplateBinding YesButtonContent}" />
                                             <Button Grid.Column="1" x:Name="PART_NoButton" MinWidth="65" Margin="6,0,0,0" Visibility="Collapsed" Content="{TemplateBinding NoButtonContent}" />
                                             <Button Grid.Column="2" x:Name="PART_OkButton" MinWidth="65" Margin="6,0,0,0" Visibility="Collapsed" Content="{TemplateBinding OkButtonContent}" />
-                                            <Button Grid.Column="3" x:Name="PART_CancelButton" MinWidth="65" Margin="6,0,0,0" Visibility="Collapsed" Content="{TemplateBinding CancelButtonContent}" />
+                                            <Button Grid.Column="3" x:Name="PART_CancelButton" MinWidth="65" Margin="6,0,0,0" Visibility="Collapsed" Content="{TemplateBinding CancelButtonContent}" IsCancel="True" />
                                         </Grid>
                                     </Grid>
                                 </Border>