Avalonia/src/Avalonia.Themes.Default/TreeView.xaml

<Style xmlns="https://github.com/avaloniaui" Selector="TreeView">
  <Setter Property="Background" Value="Transparent"/>
  <Setter Property="BorderBrush" Value="{StyleResource ThemeBorderMidBrush}"/>
  <Setter Property="BorderThickness" Value="{StyleResource ThemeBorderThickness}"/>
  <Setter Property="Padding" Value="4"/>
  <Setter Property="Template">
    <ControlTemplate>
      <Border BorderBrush="{TemplateBinding BorderBrush}"
              BorderThickness="{TemplateBinding BorderThickness}">
        <ScrollViewer CanScrollHorizontally="True" Background="{TemplateBinding Background}">
          <ItemsPresenter Name="PART_ItemsPresenter"
                          Items="{TemplateBinding Items}"
                          ItemsPanel="{TemplateBinding ItemsPanel}"
                          Margin="{TemplateBinding Padding}"
                          MemberSelector="{TemplateBinding MemberSelector}"/>
        </ScrollViewer>
      </Border>
    </ControlTemplate>
  </Setter>
</Style>