Avalonia/src/Avalonia.Themes.Default/TreeViewItem.xaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:converters="clr-namespace:Avalonia.Controls.Converters;assembly=Avalonia.Controls">
    <Style Selector="TreeViewItem">
        <Style.Resources>
            <converters:MarginMultiplierConverter Indent="16" Left="True" x:Key="LeftMarginConverter" />
        </Style.Resources>
        <Setter Property="Padding" Value="2"/>
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="Template">
            <ControlTemplate>
                <StackPanel>
                    <Border Name="SelectionBorder"
                            Focusable="True"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            CornerRadius="{TemplateBinding CornerRadius}"
                            TemplatedControl.IsTemplateFocusTarget="True">
                        <Grid Name="PART_Header"
                              ColumnDefinitions="16, *"
                              Margin="{TemplateBinding Level, Mode=OneWay, Converter={StaticResource LeftMarginConverter}}" >
                            <ToggleButton Name="expander"
                                          Focusable="False"
                                          IsChecked="{TemplateBinding IsExpanded, Mode=TwoWay}"/>
                            <ContentPresenter Name="PART_HeaderPresenter"
                                              Focusable="False"
                                              Content="{TemplateBinding Header}"
                                              HorizontalContentAlignment="{TemplateBinding HorizontalAlignment}"
                                              Padding="{TemplateBinding Padding}"
                                              Grid.Column="1"/>
                        </Grid>
                    </Border>
                    <ItemsPresenter Name="PART_ItemsPresenter"
                                    IsVisible="{TemplateBinding IsExpanded}"
                                    Items="{TemplateBinding Items}"
                                    ItemsPanel="{TemplateBinding ItemsPanel}"/>
                </StackPanel>
            </ControlTemplate>
        </Setter>
    </Style>

    <Style Selector="TreeViewItem /template/ ToggleButton#expander">
        <Setter Property="Template">
            <ControlTemplate>
                <Border Background="Transparent"
                        Width="14"
                        Height="12"
                        HorizontalAlignment="Center"
                        VerticalAlignment="Center">
                    <Path Fill="{DynamicResource ThemeForegroundBrush}"
                          HorizontalAlignment="Center"
                          VerticalAlignment="Center"
                          Data="M 0 2 L 4 6 L 0 10 Z"/>
                </Border>
            </ControlTemplate>
        </Setter>
    </Style>

    <Style Selector="TreeViewItem /template/ ContentPresenter#PART_HeaderPresenter">
        <Setter Property="Padding" Value="2"/>
    </Style>

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

    <Style Selector="TreeViewItem:selected /template/ Border#SelectionBorder">
        <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush4}"/>
    </Style>

    <Style Selector="TreeViewItem:selected /template/ Border#SelectionBorder:focus">
        <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush3}"/>
    </Style>

    <Style Selector="TreeViewItem:selected /template/ Border#SelectionBorder:pointerover">
        <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush3}"/>
    </Style>

    <Style Selector="TreeViewItem:selected /template/ Border#SelectionBorder:pointerover:focus">
        <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush2}"/>
    </Style>

    <Style Selector="TreeViewItem /template/ ToggleButton#expander:checked">
        <Setter Property="RenderTransform">
            <RotateTransform Angle="45"/>
        </Setter>
    </Style>

    <Style Selector="TreeViewItem:empty /template/ ToggleButton#expander">
        <Setter Property="IsVisible" Value="False"/>
    </Style>
</Styles>