Fix DialogHost and DrawerHost background fadeout

Changed the grid background property for dialogs and drawers to black

Author: cjvaughter

MaterialDesignThemes.Wpf/Themes/Generic.xaml

@@ -650,12 +650,11 @@
                                     x:Name="ContentPresenter" Opacity="1"                    
                                     Content="{TemplateBinding ContentControl.Content}" ContentTemplate="{TemplateBinding ContentControl.ContentTemplate}" ContentStringFormat="{TemplateBinding ContentControl.ContentStringFormat}" />
                         </AdornerDecorator>
-                        <Grid x:Name="PART_ContentCoverGrid" Background="{x:Null}" Opacity="0" IsHitTestVisible="False" Focusable="False" />
+                        <Grid x:Name="PART_ContentCoverGrid" Background="Black" Opacity="0" IsHitTestVisible="False" Focusable="False" />
                     </Grid>
                     <ControlTemplate.Triggers>
                         <Trigger Property="IsOpen" Value="True">
                             <Setter TargetName="ContentPresenter" Property="IsEnabled" Value="False" />
-                            <Setter TargetName="PART_ContentCoverGrid" Property="Background" Value="Black" />
                             <Setter TargetName="PART_ContentCoverGrid" Property="IsHitTestVisible" Value="True" />
                         </Trigger>
                     </ControlTemplate.Triggers>
@@ -929,7 +928,7 @@
                                         x:Name="ContentPresenter" Opacity="1"                    
                                         Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}" ContentStringFormat="{TemplateBinding ContentStringFormat}" />
                             </AdornerDecorator>
-                            <Grid x:Name="PART_ContentCover" Background="{x:Null}" Opacity="0" IsHitTestVisible="False" Focusable="False" />
+                            <Grid x:Name="PART_ContentCover" Background="Black" Opacity="0" IsHitTestVisible="False" Focusable="False" />
                             <Grid>
                                 <Grid HorizontalAlignment="Left" VerticalAlignment="Stretch"
                                       x:Name="PART_LeftDrawer"