HandyControl/src/Avalonia/HandyControl_Avalonia/Themes/Styles/TreeView.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal"
                    xmlns:hc="clr-namespace:HandyControl.Controls">
    <Geometry x:Key="TreeArrow">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</Geometry>

    <ControlTheme x:Key="ExpandCollapseToggleStyle"
                  TargetType="ToggleButton">
        <Setter Property="Width"
                Value="16" />
        <Setter Property="Height"
                Value="16" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border Background="Transparent"
                        Height="16"
                        Padding="4"
                        Width="16">
                    <Path x:Name="expandPath"
                          Stretch="Uniform"
                          HorizontalAlignment="Center"
                          VerticalAlignment="Center"
                          Data="{StaticResource TreeArrow}"
                          Fill="{TemplateBinding Foreground}">
                        <Path.RenderTransform>
                            <RotateTransform Angle="-90" />
                        </Path.RenderTransform>
                    </Path>
                </Border>
            </ControlTemplate>
        </Setter>

        <Style Selector="^:checked /template/ Path#expandPath">
            <Setter Property="RenderTransform">
                <RotateTransform Angle="0" />
            </Setter>
        </Style>
    </ControlTheme>

    <ControlTheme x:Key="TreeViewItemBaseStyle"
                  TargetType="TreeViewItem">
        <Setter Property="Padding"
                Value="10,0" />
        <Setter Property="MinHeight"
                Value="{StaticResource DefaultControlHeight}" />
        <Setter Property="Background"
                Value="{DynamicResource RegionBrush}" />
        <Setter Property="BorderBrush"
                Value="Transparent" />
        <Setter Property="CornerRadius"
                Value="{Binding $self.(hc:BorderElement.CornerRadius)}" />
        <Setter Property="BorderThickness"
                Value="0" />
        <Setter Property="Margin"
                Value="0,0,0,2" />
        <Setter Property="Template">
            <ControlTemplate>
                <StackPanel>
                    <Border Name="SelectionBorder"
                            MinHeight="{TemplateBinding MinHeight}"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            CornerRadius="{TemplateBinding CornerRadius}"
                            Focusable="True"
                            Padding="{TemplateBinding Padding}"
                            TemplatedControl.IsTemplateFocusTarget="True">
                        <Grid Name="PART_Header"
                              Margin="{TemplateBinding Level, Mode=OneWay, Converter={StaticResource LeftMarginConverter}}"
                              ColumnDefinitions="16, *">
                            <ToggleButton Name="PART_ExpandCollapseChevron"
                                          Focusable="False"
                                          Background="Transparent"
                                          IsChecked="{TemplateBinding IsExpanded, Mode=TwoWay}"
                                          Theme="{StaticResource ExpandCollapseToggleStyle}" />
                            <ContentPresenter Name="PART_HeaderPresenter"
                                              Grid.Column="1"
                                              Background="Transparent"
                                              VerticalAlignment="Center"
                                              HorizontalContentAlignment="{TemplateBinding HorizontalAlignment}"
                                              Content="{TemplateBinding Header}"
                                              ContentTemplate="{TemplateBinding HeaderTemplate}"
                                              Focusable="False" />
                        </Grid>
                    </Border>
                    <ItemsPresenter Name="PART_ItemsPresenter"
                                    IsVisible="{TemplateBinding IsExpanded}"
                                    ItemsPanel="{TemplateBinding ItemsPanel}" />
                </StackPanel>
            </ControlTemplate>
        </Setter>

        <Style Selector="^ /template/ Border#SelectionBorder:pointerover">
            <Setter Property="Background"
                    Value="{DynamicResource SecondaryRegionBrush}" />
        </Style>

        <Style Selector="^:selected">
            <Style Selector="^ /template/ Border#SelectionBorder, ^ /template/ Border#SelectionBorder:pointerover">
                <Setter Property="Background"
                        Value="{DynamicResource PrimaryBrush}" />
            </Style>
            <Style Selector="^ /template/ ContentPresenter#PART_HeaderPresenter">
                <Setter Property="Foreground"
                        Value="{DynamicResource TextIconBrush}" />
            </Style>
            <Style Selector="^ /template/ ToggleButton#PART_ExpandCollapseChevron">
                <Setter Property="Foreground"
                        Value="{DynamicResource TextIconBrush}" />
            </Style>
        </Style>

        <Style Selector="^:empty /template/ ToggleButton#PART_ExpandCollapseChevron">
            <Setter Property="IsVisible"
                    Value="False" />
        </Style>

        <Style Selector="^:disabled">
            <Setter Property="Opacity"
                    Value="0.4" />
        </Style>
    </ControlTheme>

    <ControlTheme x:Key="TreeViewBaseStyle"
                  TargetType="TreeView">
        <Setter Property="Background"
                Value="{DynamicResource RegionBrush}" />
        <Setter Property="BorderBrush"
                Value="{DynamicResource BorderBrush}" />
        <Setter Property="BorderThickness"
                Value="1" />
        <Setter Property="hc:BorderElement.CornerRadius"
                Value="{StaticResource DefaultCornerRadius}" />
        <Setter Property="CornerRadius"
                Value="{Binding $self.(hc:BorderElement.CornerRadius)}" />
        <Setter Property="Padding"
                Value="2,2,2,0" />
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility"
                Value="Auto" />
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility"
                Value="Auto" />
        <Setter Property="ScrollViewer.IsScrollChainingEnabled"
                Value="True" />
        <Setter Property="ItemContainerTheme"
                Value="{StaticResource TreeViewItemBaseStyle}" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        Background="{TemplateBinding Background}"
                        CornerRadius="{TemplateBinding CornerRadius}">
                    <ScrollViewer AllowAutoHide="{TemplateBinding (ScrollViewer.AllowAutoHide)}"
                                  BringIntoViewOnFocusChange="{TemplateBinding (ScrollViewer.BringIntoViewOnFocusChange)}"
                                  HorizontalScrollBarVisibility="{TemplateBinding (ScrollViewer.HorizontalScrollBarVisibility)}"
                                  IsScrollChainingEnabled="{TemplateBinding (ScrollViewer.IsScrollChainingEnabled)}"
                                  IsDeferredScrollingEnabled="{TemplateBinding (ScrollViewer.IsDeferredScrollingEnabled)}"
                                  VerticalScrollBarVisibility="{TemplateBinding (ScrollViewer.VerticalScrollBarVisibility)}"
                                  VerticalSnapPointsType="{TemplateBinding (ScrollViewer.VerticalSnapPointsType)}"
                                  HorizontalSnapPointsType="{TemplateBinding (ScrollViewer.HorizontalSnapPointsType)}">
                        <ItemsPresenter Name="PART_ItemsPresenter"
                                        Margin="{TemplateBinding Padding}"
                                        ItemsPanel="{TemplateBinding ItemsPanel}" />
                    </ScrollViewer>
                </Border>
            </ControlTemplate>
        </Setter>
    </ControlTheme>

    <ControlTheme x:Key="{x:Type TreeView}"
                  BasedOn="{StaticResource TreeViewBaseStyle}"
                  TargetType="TreeView" />

    <ControlTheme x:Key="TreeViewItemBaseStyle.Small"
                  BasedOn="{StaticResource TreeViewItemBaseStyle}"
                  TargetType="TreeViewItem">
        <Setter Property="Padding"
                Value="6,0" />
        <Setter Property="MinHeight"
                Value="24" />
    </ControlTheme>

    <ControlTheme x:Key="TreeView.Small"
                  BasedOn="{StaticResource TreeViewBaseStyle}"
                  TargetType="TreeView">
        <Setter Property="ItemContainerTheme"
                Value="{StaticResource TreeViewItemBaseStyle.Small}" />
    </ControlTheme>
</ResourceDictionary>