Fix Expander Dark theme issue

Fix issue #274

Author: l1pton17

MainDemo.Wpf/Expander.xaml

@@ -16,8 +16,10 @@
                 <Expander.Header>
                     <TextBlock Text="Expander Example 1" />
                 </Expander.Header>
-                <Border Background="{StaticResource MaterialDesignBackground}" CornerRadius="5" Padding="5">
-                    <StackPanel Orientation="Vertical">
+                <Border Background="{DynamicResource MaterialDesignBody}"
+                        CornerRadius="5" Padding="5">
+                    <StackPanel Orientation="Vertical"
+                                TextBlock.Foreground="{DynamicResource MaterialDesignPaper}">
                         <TextBlock Text="A short text to proof functionality." />
                         <TextBlock Margin="0,50,0,0" Text="A short text to proof functionality." />
                     </StackPanel>

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Expander.xaml

@@ -106,6 +106,7 @@
 
     <Style x:Key="MaterialDesignExpander" TargetType="{x:Type Expander}">
         <Setter Property="Background" Value="{StaticResource MaterialDesignPaper}" />
+        <Setter Property="Foreground" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}, Path=(TextElement.Foreground)}"/>
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type Expander}">
@@ -173,9 +174,10 @@
                                     <ColumnDefinition Width="100*" />
                                 </Grid.ColumnDefinitions>
                                 <ToggleButton IsChecked="{Binding Path=IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
-                                              Foreground="{Binding Path=Foreground, RelativeSource={RelativeSource TemplatedParent}}" OverridesDefaultStyle="True"
+                                              OverridesDefaultStyle="True"
                                               Style="{StaticResource MaterialDesignExpanderToggleButton}" />
-                                <ToggleButton Grid.Column="1" IsChecked="{Binding Path=IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
+                                <ToggleButton Grid.Column="1"
+                                              IsChecked="{Binding Path=IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                                               Style="{StaticResource MaterialDesignExpanderHeaderToggleButton}">
                                     <ContentPresenter ContentSource="Header" HorizontalAlignment="Stretch" RecognizesAccessKey="True" VerticalAlignment="Center" />
                                 </ToggleButton>