Unable to change the color of chevrons in ScrollBar

[Angelore]

I've noticed that it is not possible to change the color of chevrons in the scrollbar, even though it is supposed to allow doing this via resources.
I've copied the whole control theme to do a small investigation. Very simple sample here: ScrollBarChevron.zip

What I've found out is that setting an attached property in a control theme doesn't work, or at the very least doesn't work the way the author of the default theme intended it to work.

Starting with the testing preview that is included with the component:

<Window.Resources>
    <SolidColorBrush x:Key="ScrollBarButtonArrowForeground" Color="Red" Opacity="1"/>
    <SolidColorBrush x:Key="ScrollBarButtonArrowForegroundDisabled" Color="Red" Opacity="1"/>
</Window.Resources>

<Border Padding="20">
  <StackPanel Orientation="Horizontal" Spacing="20">
    <StackPanel Spacing="20">
      <ScrollBar Orientation="Horizontal" AllowAutoHide="False" VerticalAlignment="Center" Width="100"/>
      <ScrollBar Orientation="Horizontal" AllowAutoHide="False" VerticalAlignment="Center" Width="100" IsEnabled="False"/>
      <ScrollBar Orientation="Horizontal" AllowAutoHide="True" VerticalAlignment="Center" Width="100"/>
      <ScrollBar Orientation="Horizontal" AllowAutoHide="True" VerticalAlignment="Center" Width="100" IsEnabled="False"/>
    </StackPanel>
    <ScrollBar Orientation="Vertical" AllowAutoHide="False" HorizontalAlignment="Center" Height="100"/>
    <ScrollBar Orientation="Vertical" AllowAutoHide="False" HorizontalAlignment="Center" Height="100" IsEnabled="False"/>
    <ScrollBar Orientation="Vertical" AllowAutoHide="True" HorizontalAlignment="Center" Height="100"/>
    <ScrollBar Orientation="Vertical" AllowAutoHide="True" HorizontalAlignment="Center" Height="100" IsEnabled="False"/>
  </StackPanel>
</Border>

Which results in this:

Furthermore, setting this property on anything other than the PathIcon that is being passed into the RepeatButton does nothing.
As per the link above, the control theme for it looks like this:

<ControlTheme x:Key="FluentScrollBarLineButton" TargetType="RepeatButton">
    ...
    <Setter Property="TextElement.Foreground" Value="{DynamicResource ScrollBarButtonArrowForeground}"/>
    <Setter Property="Template">
        <ControlTemplate>
            <ContentPresenter x:Name="PART_ContentPresenter"
                Background="{DynamicResource ScrollBarButtonBackground}"
                BorderBrush="{DynamicResource ScrollBarButtonBorderBrush}"
                Content="{TemplateBinding Content}"/>
            </ControlTemplate>
    </Setter>
    ...
</ControlTheme>

As an aside, I've tried the following on the PathIcon itself. Naturally if one was going to do this, they wouldn't have bothered with the TextElement property in the first place.

<!-- Foreground="{TemplateBinding Foreground}" --> -- Doesn't work.
<!-- Foreground="{DynamicResource ScrollBarButtonArrowForegroundDisabled}" --> Works.
<!-- Foreground="{Binding $parent.Foreground}" --> Works.

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Back to the topic at hand: Is this a mistake in the code of the control, or is this a bug?