WPF - Change background color of MenuItem when the page it links to is active (not when MouseOver)

Question

I'd like to change the background color of a MenuItem in WPF if the page it links to is the page I'm currently on. I've already figured out how to change a MenuItem upon MouseOver, and figured it would be a similar process for an active page. Unfortunately all the links I've seen here when searching for this problem solve the issue for MouseOver, not for the active page.

I was thinking along the lines of setting the "IsFocused" trigger in the code-behind, but that doesn't seem to work because IsFocused is readonly, and I'm not sure if that's the correct property to set anyways.

<StackPanel Name="spMainNav">
    <StackPanel.Resources>

        <ControlTemplate x:Key="VsMenuSub" TargetType="MenuItem">
            <StackPanel>
                <Label Content="{TemplateBinding Header}"  Foreground="White" />

                <StackPanel.Style>
                    <Style TargetType="StackPanel">
                        <Style.Triggers>
                            <Trigger Property="IsMouseOver" Value="True">
                                <Setter Property="Background" Value="Red" />
                            </Trigger>

                            <!--Don't know where to set this in code-behind?-->
                            <Trigger Property="IsFocused" Value="True">
                                <Setter Property="Background" Value="DarkRed" />
                            </Trigger>
                        </Style.Triggers>
                    </Style>
                </StackPanel.Style>
            </StackPanel>
        </ControlTemplate>
            
    </StackPanel.Resources>

    <MenuItem Name="ProjectList" Header="Project List" Template="{StaticResource VsMenuSub}" Click="ProjectList_Click" />
    <MenuItem Header="About" Template="{StaticResource VsMenuSub}" Click="About_Click" />
    <MenuItem Header="Exit" Template="{StaticResource VsMenuSub}" Click="Exit_Click" />
</StackPanel>

I'm able to modify the header text of an active link (appending two asterisks to the beginning). I thought I could set the background color this way, but I can't figure out how to do it. Setting the Background field of the MenuItem to a brush doesn't work.

Public Shared Sub HighlightMenuItem(sender As MenuItem)

    Dim sp As StackPanel = sender.Parent

    Dim bc As New BrushConverter
    sp.Background = bc.ConvertFrom("#36637C")

    Dim thisMI As MenuItem = sender
    Dim controlCount As Integer = VisualTreeHelper.GetChildrenCount(sp)

    For i As Integer = 0 To controlCount - 1
        Dim c = VisualTreeHelper.GetChild(sp, i)

        If TypeOf c Is MenuItem Then
            Dim mi As MenuItem = c

            If mi.Header = thisMI.Header Then
                mi.Header = "** " & mi.Header

                ' This doesn't work
                mi.Background = Brushes.Blue
            End If

        End If
    Next
End Sub