HandyControl/src/Shared/HandyControl_Shared/Themes/Styles/Base/TreeViewBaseStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:hc="clr-namespace:HandyControl.Controls">

    <PathGeometry x:Key="TreeArrow" Figures="M508.893248 762.931659a65.039498 65.039498 0 0 1-46.420659-19.485839L97.125746 371.004022c-25.143018-25.634319-24.752868-66.816849 0.881451-91.959868 25.648769-25.164693 66.809624-24.745643 91.959867 0.881451l318.933409 325.125238 318.933408-325.125238a65.025048 65.025048 0 0 1 92.841318 91.078417L555.313907 743.44582a65.025048 65.025048 0 0 1-46.420659 19.485839z"/>
    <Style x:Key="ExpandCollapseToggleStyle" TargetType="ToggleButton">
        <Setter Property="Focusable" Value="False"/>
        <Setter Property="Width" Value="16"/>
        <Setter Property="Height" Value="16"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="ToggleButton">
                    <Border Background="Transparent" Height="16" Padding="4" Width="16">
                        <Path x:Name="ExpandPath" RenderTransformOrigin="0.5,0.5" Stretch="Uniform" Data="{StaticResource TreeArrow}" Fill="{Binding Foreground, RelativeSource={RelativeSource AncestorType=TreeViewItem}}">
                            <Path.RenderTransform>
                                <RotateTransform Angle="-90"/>
                            </Path.RenderTransform>
                        </Path>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsChecked" Value="True">
                            <Setter Property="RenderTransform" TargetName="ExpandPath">
                                <Setter.Value>
                                    <RotateTransform Angle="0"/>
                                </Setter.Value>
                            </Setter>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style x:Key="TreeViewItemBaseStyle" BasedOn="{StaticResource BaseStyle}" TargetType="TreeViewItem">
        <Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisualRadius0Margin0}"/>
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="Padding" Value="10,0"/>
        <Setter Property="BorderThickness" Value="0"/>
        <Setter Property="MinHeight" Value="{StaticResource DefaultControlHeight}"/>
        <Setter Property="Margin" Value="0,0,0,2" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="TreeViewItem">
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition MinHeight="{TemplateBinding MinHeight}"/>
                            <RowDefinition Height="Auto"/>
                        </Grid.RowDefinitions>
                        <Border x:Name="Bd" CornerRadius="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true">
                            <DockPanel LastChildFill="True" Margin="{Binding Converter={StaticResource TreeViewItemMarginConverter}, RelativeSource={RelativeSource TemplatedParent}}">
                                <ToggleButton x:Name="Expander" ClickMode="Press" IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ExpandCollapseToggleStyle}"/>
                                <ContentPresenter VerticalAlignment="Center" x:Name="PART_Header" ContentSource="Header" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                            </DockPanel>
                        </Border>
                        <ItemsPresenter x:Name="ItemsHost" Grid.Row="1"/>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsMouseOver" Value="true" SourceName="Bd">
                            <Setter Property="Background" TargetName="Bd" Value="{DynamicResource SecondaryRegionBrush}"/>
                        </Trigger>
                        <Trigger Property="IsExpanded" Value="false">
                            <Setter Property="Visibility" TargetName="ItemsHost" Value="Collapsed"/>
                        </Trigger>
                        <Trigger Property="HasItems" Value="false">
                            <Setter Property="Visibility" TargetName="Expander" Value="Hidden"/>
                        </Trigger>
                        <Trigger Property="IsSelected" Value="true">
                            <Setter Property="Background" TargetName="Bd" Value="{DynamicResource PrimaryBrush}"/>
                            <Setter Property="Foreground" Value="{DynamicResource TextIconBrush}"/>
                        </Trigger>
                        <MultiTrigger>
                            <MultiTrigger.Conditions>
                                <Condition Property="IsSelected" Value="true"/>
                                <Condition Property="IsSelectionActive" Value="false"/>
                            </MultiTrigger.Conditions>
                            <Setter Property="Background" TargetName="Bd" Value="{DynamicResource DarkDefaultBrush}"/>
                            <Setter Property="Foreground" Value="{DynamicResource PrimaryTextBrush}"/>
                        </MultiTrigger>
                        <Trigger Property="IsEnabled" Value="false">
                            <Setter Property="Opacity" Value=".4" />
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style x:Key="TreeViewBaseStyle" TargetType="TreeView" BasedOn="{StaticResource BaseStyle}">
        <Setter Property="Focusable" Value="False"/>
        <Setter Property="Background" Value="{DynamicResource RegionBrush}"/>
        <Setter Property="BorderBrush" Value="{DynamicResource BorderBrush}"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="Padding" Value="2,2,2,0" />
        <Setter Property="hc:BorderElement.CornerRadius" Value="{StaticResource DefaultCornerRadius}" />
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.PanningMode" Value="Both"/>
        <Setter Property="hc:ScrollViewer.IsInertiaEnabled" Value="False"/>
        <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
        <Setter Property="VerticalContentAlignment" Value="Center"/>
        <Setter Property="VirtualizingPanel.IsVirtualizing" Value="True"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="TreeView">
                    <Border x:Name="Bd" CornerRadius="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" SnapsToDevicePixels="true">
                        <hc:ToggleBlock IsChecked="{Binding HasItems,RelativeSource={RelativeSource TemplatedParent},Mode=OneWay}" VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch">
                            <hc:ToggleBlock.CheckedContent>
                                <hc:ScrollViewer x:Name="_tv_scrollviewer_" IsInertiaEnabled="{Binding Path=(hc:ScrollViewer.IsInertiaEnabled),RelativeSource={RelativeSource TemplatedParent}}" Focusable="false" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
                                    <ItemsPresenter/>
                                </hc:ScrollViewer>
                            </hc:ToggleBlock.CheckedContent>
                            <hc:ToggleBlock.UnCheckedContent>
                                <hc:Empty/>
                            </hc:ToggleBlock.UnCheckedContent>
                        </hc:ToggleBlock>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="VirtualizingPanel.IsVirtualizing" Value="true">
                            <Setter Property="CanContentScroll" TargetName="_tv_scrollviewer_" Value="true"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <Trigger Property="VirtualizingPanel.IsVirtualizing" Value="true">
                <Setter Property="ItemsPanel">
                    <Setter.Value>
                        <ItemsPanelTemplate>
                            <VirtualizingStackPanel/>
                        </ItemsPanelTemplate>
                    </Setter.Value>
                </Setter>
            </Trigger>
        </Style.Triggers>
    </Style>

</ResourceDictionary>