aistudio-wpf-diagram/AIStudio.Wpf.DiagramDesigner/Styles/Expander.xaml

<ResourceDictionary 
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Shared.xaml" />
    </ResourceDictionary.MergedDictionaries>

    <!-- SimpleStyles: Expander -->

    <ControlTemplate x:Key="AIStudio.DiagramApp.Styles.ToggleButton.Expander" TargetType="ToggleButton">
        <Border
      Name="Border" 
      CornerRadius="2,0,0,0"
      Background="Transparent"
      BorderBrush="{StaticResource NormalBorderBrush}"
      BorderThickness="0,0,0,0">
            <Path 
        Name="Arrow"
        Fill="{StaticResource GlyphBrush}"
        HorizontalAlignment="Center"
        VerticalAlignment="Center"
        Data="M 0 0 L 4 4 L 8 0 Z"/>
        </Border>
        <ControlTemplate.Triggers>
            <Trigger Property="ToggleButton.IsMouseOver" Value="true">
                <Setter TargetName="Border" Property="Background"
                Value="{StaticResource DarkBrush}" />
            </Trigger>
            <Trigger Property="IsPressed" Value="true">
                <Setter TargetName="Border" Property="Background"
                Value="{StaticResource PressedBrush}" />
            </Trigger>
            <Trigger Property="IsChecked" Value="true">
                <Setter TargetName="Arrow" Property="Data"
                Value="M 0 4 L 4 0 L 8 4 Z" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="False">
                <Setter TargetName="Border" Property="Background"
                Value="{StaticResource DisabledBackgroundBrush}" />
                <Setter TargetName="Border" Property="BorderBrush"
                Value="{StaticResource DisabledBorderBrush}" />
                <Setter Property="Foreground"
                Value="{StaticResource DisabledForegroundBrush}"/>
                <Setter TargetName="Arrow" Property="Fill"
                Value="{StaticResource DisabledForegroundBrush}" />
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <Style TargetType="Expander">
        <Setter Property="FontFamily" Value="SegoeUI"/>
        <Setter Property="FontSize" Value="12"/>
        <Setter Property="Foreground" Value="#4C4C4C"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="Expander">
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Name="ContentRow" Height="0"/>
                        </Grid.RowDefinitions>
                        <Border 
              Name="Border" 
              Grid.Row="0" 
              Background="{StaticResource LightBrush}"
              BorderBrush="{StaticResource NormalBorderBrush}"
              BorderThickness="1" 
              CornerRadius="2,2,0,0" >
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="20" />
                                    <ColumnDefinition Width="*" />
                                </Grid.ColumnDefinitions>
                                <ToggleButton
                  IsChecked="{Binding Path=IsExpanded,Mode=TwoWay,
                                      RelativeSource={RelativeSource TemplatedParent}}"
                  OverridesDefaultStyle="True" 
                  Template="{StaticResource AIStudio.DiagramApp.Styles.ToggleButton.Expander}" 
                  Background="{StaticResource NormalBrush}" />
                                <ContentPresenter 
                  Grid.Column="1"
                  Margin="4" 
                  ContentSource="Header" 
                  RecognizesAccessKey="True" />
                            </Grid>
                        </Border>
                        <Border 
              Name="Content" 
              Grid.Row="1" 
              Background="{StaticResource WindowBackgroundBrush}"
              BorderBrush="{StaticResource SolidBorderBrush}" 
              BorderThickness="1,0,1,1" 
              CornerRadius="0,0,2,2" >
                            <ContentPresenter Margin="4" />
                        </Border>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsExpanded" Value="True">
                            <Setter TargetName="ContentRow" Property="Height"
                      Value="{Binding ElementName=Content,Path=DesiredHeight}" />
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter TargetName="Border" Property="Background"
                      Value="{StaticResource DisabledBackgroundBrush}" />
                            <Setter TargetName="Border" Property="BorderBrush"
                      Value="{StaticResource DisabledBorderBrush}" />
                            <Setter Property="Foreground"
                      Value="{StaticResource DisabledForegroundBrush}"/>
                        </Trigger>

                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>