Стиль к https://ru.stackoverflow.com/a/738993/10105

MyCoolExpanderStyle.xaml

<Style x:Key="ExpanderHeaderFocusVisual">
    <Setter Property="Control.Template">
        <Setter.Value>
            <ControlTemplate>
                <Border>
                    <Rectangle Margin="0" SnapsToDevicePixels="true" Stroke="Black" StrokeThickness="1" StrokeDashArray="1 2"/>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<Style x:Key="ExpanderDownHeaderStyle" TargetType="{x:Type ToggleButton}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Border Padding="{TemplateBinding Padding}" Background="Transparent">
                    <ContentPresenter HorizontalAlignment="Left" RecognizesAccessKey="True" VerticalAlignment="Center"/>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<Style x:Key="MyCoolExpanderStyle" TargetType="{x:Type Expander}">
    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlLightBrushKey}}"/>
    <Setter Property="Background" Value="Coral"/>
    <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
    <Setter Property="VerticalContentAlignment" Value="Stretch"/>
    <Setter Property="BorderBrush" Value="Transparent"/>
    <Setter Property="BorderThickness" Value="1"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Expander}">
                <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="true">
                    <DockPanel>
                        <ToggleButton x:Name="HeaderSite" ContentTemplate="{TemplateBinding HeaderTemplate}" ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}" Content="{TemplateBinding Header}" DockPanel.Dock="Top" Foreground="{TemplateBinding Foreground}" FontWeight="{TemplateBinding FontWeight}" FocusVisualStyle="{StaticResource ExpanderHeaderFocusVisual}" FontStyle="{TemplateBinding FontStyle}" FontStretch="{TemplateBinding FontStretch}" FontSize="{TemplateBinding FontSize}" FontFamily="{TemplateBinding FontFamily}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" MinWidth="0" MinHeight="0" Padding="{TemplateBinding Padding}" Style="{StaticResource ExpanderDownHeaderStyle}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
                        <Grid Background="Black" x:Name="ExpandSite" DockPanel.Dock="Bottom" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" Visibility="Collapsed" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                            <Canvas>
                                <Path Data="M 20,0 L 50,0 L35,10 z" StrokeThickness="0" Fill="{TemplateBinding Background}" Stretch="None"/>
                            </Canvas>
                            <ContentPresenter Focusable="false"/>
                        </Grid>
                    </DockPanel>
                </Border>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsExpanded" Value="true">
                        <Setter Property="Visibility" TargetName="ExpandSite" Value="Visible"/>
                    </Trigger>
                    <Trigger Property="IsEnabled" Value="false">
                        <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>