Using an Existing theme to create a completely new theme

I think I am nearly there. If I can understand this last thing I believe I can finish....

My problem is this: The RoyalLight theme using > and a down pointing version of > for the collapsed and expanded indicators in the XamTreeGrid. I have managed to get all the colors, cell borders, etc that I want to change implemented but I cannot figure out where these graphics are coming from. It would seem that the areas in DataPresenterRoyalLight_Express starting around line 4721 would be the target area. Specifically starting at line 4788 ****ExpandCollapseToggle Style*** this code would seem to imply the style:

   <!--  ********************************** ExpandCollapseToggle Style **********************************  -->
    <Style x:Key="ExpandCollapseToggleStyle" TargetType="{x:Type  ToggleButton}">
        <Setter Property="Focusable" Value="False" />
        <Setter Property="Width" Value="22" />
        <Setter Property="Height" Value="22" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ToggleButton}">
                    <Border Background="{StaticResource TransparentBrush}" Padding="0">
                        <Grid>
                            <Path x:Name="ExpansionIndicatorCollapsed"
                                  Width="5"
                                  Height="8"
                                  HorizontalAlignment="Center"
                                  VerticalAlignment="Center"
                                  Data="M60,40L60,20 70,30z"
                                  Fill="{StaticResource ExpansionIndicatorGlyphBackgroundBrush}"
                                  RenderTransformOrigin="0.5,0.5"
                                  Stretch="Fill"
                                  Stroke="{StaticResource ExpansionIndicatorGlyphBorderBrush}"
                                  StrokeThickness="1"
                                  UseLayoutRounding="False" />
                            <Path x:Name="ExpansionIndicatorExpanded"
                                  Width="6"
                                  Height="6"
                                  HorizontalAlignment="Center"
                                  VerticalAlignment="Center"
                                  Data="M0.5,6.4997156 L3.3283137,3.6714226 3.6714667,3.6714429 3.6714465,3.3282926 6.4997594,0.50000001 6.500113,6.5000692 z"
                                  Fill="{StaticResource ExpansionIndicatorGlyphExpandedBackgroundBrush}"
                                  RenderTransformOrigin="0.5,0.5"
                                  Stretch="Fill"
                                  Stroke="{StaticResource ExpansionIndicatorGlyphBorderBrush}"
                                  StrokeThickness="1"
                                  UseLayoutRounding="False"
                                  Visibility="Collapsed" />
                        </Grid>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsMouseOver" Value="True">
                            <Setter TargetName="ExpansionIndicatorCollapsed" Property="Stroke" Value="{StaticResource ExpansionIndicatorGlyphHoverBackgroundBrush}" />
                            <Setter TargetName="ExpansionIndicatorCollapsed" Property="Fill" Value="{StaticResource ExpansionIndicatorGlyphHoverBackgroundBrush}" />
                            <Setter TargetName="ExpansionIndicatorExpanded" Property="Stroke" Value="{StaticResource ExpansionIndicatorGlyphHoverBackgroundBrush}" />
                            <Setter TargetName="ExpansionIndicatorExpanded" Property="Fill" Value="{StaticResource ExpansionIndicatorGlyphHoverBackgroundBrush}" />
                        </Trigger>
                        <Trigger Property="IsChecked" Value="True">
                            <Setter TargetName="ExpansionIndicatorExpanded" Property="Visibility" Value="Visible" />
                            <Setter TargetName="ExpansionIndicatorCollapsed" Property="Visibility" Value="Collapsed" />
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

Alas no amount of fiddling affects my outcome

I get this

but I want this

That is as far as the > versus + is concerned.

I am looking in the wrong place undoubtedly. But I have worked the entire DataPresenter chain for the RoyalLight theme so I am at a loss.

If I could crack this nut my up coming demo would be perfect.

Thx

Rick