Replace part of default template in WPF

is there any "best practice" way to replace a part of the default template. The current use case is a treeview. As default, the treeview has this small triangle shapes to expand and collapse.

I know how to replace these if I replace the whole control template, as shown in the code below. I am not sure if there is a way to "keep all default, just change XY". Its not a style, I basically need to replace a part of an existing control template.

To illustrate, take a look at the following XAML. The first smaller block is the relevant XAML I want to be able to adapt.

The bigger second and third part are basically a copy of the default templates, only to administer the "changed" part from the beginning.

Is there a better way to do this, saving the long and confusing XAML in the second half?

        <ResourceDictionary 
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >





  <Style x:Key="ExpandCollapseToggleStyle" TargetType="ToggleButton">
    <Setter Property="Focusable" Value="False"/>
     <Setter Property="Template">
      <Setter.Value>
        <ControlTemplate TargetType="ToggleButton">
          <Grid
            Width="15"
            Height="13"
            Background="Transparent">
            <Path x:Name="ExpandPath"
              HorizontalAlignment="Left" 
              VerticalAlignment="Center" 
              Margin="1,1,1,1"
              Fill="Black"
              Data="M 4 0 L 8 4 L 4 8 Z"/>
          </Grid>
          <ControlTemplate.Triggers>
            <Trigger Property="IsChecked"
                 Value="True">
              <Setter Property="Data"
                  TargetName="ExpandPath"
                  Value="M 0 4 L 8 4 L 4 8 Z"/>
            </Trigger>
          </ControlTemplate.Triggers>
        </Cont