Fix #1709 (#1711)

* Fix #1709

The select all button had a non dynamic background, which made theming difficult. With this change, the button behaves more in line with other elements.

* Adding example in demo app

Co-authored-by: Kevin Bost <[email protected]>

Author: w-syss

MainDemo.Wpf/Grids.xaml

@@ -19,7 +19,7 @@
             <TextBlock>Custom Columns</TextBlock>
             <smtx:XamlDisplay Key="grids_1">
                 <DataGrid Margin="0 8 0 0" ItemsSource="{Binding Items3}" CanUserSortColumns="True" CanUserAddRows="False" AutoGenerateColumns="False"
-                          materialDesign:DataGridAssist.CellPadding="13 8 8 8" materialDesign:DataGridAssist.ColumnHeaderPadding="8">
+                          materialDesign:DataGridAssist.CellPadding="13 8 8 8" materialDesign:DataGridAssist.ColumnHeaderPadding="8" HeadersVisibility="All">
                     <DataGrid.Columns>
                         <DataGridCheckBoxColumn Binding="{Binding IsSelected}" 
                                                 ElementStyle="{StaticResource MaterialDesignDataGridCheckBoxColumnStyle}"

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.DataGrid.xaml

@@ -128,8 +128,8 @@
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type Button}">
                     <Grid>
-                        <Rectangle x:Name="Border" Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" SnapsToDevicePixels="True"/>
-                        <Polygon x:Name="Arrow" Fill="Black" HorizontalAlignment="Right" Margin="8,8,3,3" Opacity="0.15" Points="0,10 10,10 10,0" Stretch="Uniform" VerticalAlignment="Bottom"/>
+                        <Rectangle x:Name="Border" Fill="Transparent" SnapsToDevicePixels="True"/>
+                        <Polygon x:Name="Arrow" Fill="{DynamicResource MaterialDesignBody}" HorizontalAlignment="Right" Margin="8,8,3,3" Opacity="0.5" Points="0,10 10,10 10,0" Stretch="Uniform" VerticalAlignment="Bottom"/>
                     </Grid>
                     <ControlTemplate.Triggers>
                         <Trigger Property="IsMouseOver" Value="True">