WPF: How to have custom button template that uses system's button background by default?

Question

Hi, I am creating a style that changes a ControlTemplate of a Button (it actually adds arrow on the right, so that the button looks like dropdown button (which is missing in wpf)).

Inside of template, I put the actual button (because I need the result to still be like button - I could change only ContentTemplate, but I need to display the actual content together with arrow, and putting ContentPresenter inside ContentTemplate yields stack overflow - wpf just expands the template into the presenter and goes on and on), together with arrow.

My problem is that I would like the user to be able to change button's background, so I need to bind Background="{TemplateBinding Background}". With this, the user would have to always provide the background, otherwise it would be null. To prevent this, I understand I need to provide default value for the background, so I added <Setter Property="Background" Value="default Background goes here"/>

The question is, what should be a value of this setter? I assume that wpf's built-in styles are loaded automatically for every application, and I have looked into aero.normalcolor.xaml, where they provide ButtonNormalBackground style, and later use it as default button style <Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/> (aero.normalcolor.xaml, line 47).

Exactly what I wanted, however, when I use this setter, the static resource is not found. I then changed it to DynamicResource, but in that case, the button's background stays transparent - the ButtonNormalBackground isn't used!

How should I provide system's button look as default value? Thank you!

PS: dtto for BorderBrush, BorderThickness - geez, it seems that I am not allowed the tricks that WPF authors use all the time :/

My style:

<Style TargetType="{x:Type ToggleButton}" x:Key="DropDownArrowStyle">
 <Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
 <Setter Property="ToggleButton.Template">
  <Setter.Value>
   <ControlTemplate TargetType="{x:Type ToggleButton}">
    <ToggleButton 
     x:Name="PART_ToggleButton"
     Margin="{TemplateBinding Margin}"
     Padding="{TemplateBinding Padding}"
     HorizontalAlignment="Stretch"
     VerticalAlignment="Stretch"
     HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
     VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
     IsEnabled="{TemplateBinding IsEnabled}"
     IsChecked="{Binding IsChecked, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
     ContextMenu="{TemplateBinding ContextMenu}"
     Background="{TemplateBinding Background}"
     BorderBrush="{TemplateBinding BorderBrush}"
     BorderThickness="{TemplateBinding BorderThickness}"
     Foreground="{TemplateBinding Foreground}"
     FocusVisualStyle="{TemplateBinding FocusVisualStyle}"
     ClickMode="{TemplateBinding ClickMode}"
     >
     <Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
      <Grid.ColumnDefinitions>
       <ColumnDefinition Width="*" />
       <ColumnDefinition Width="Auto" />
      </Grid.ColumnDefinitions>
      <ContentPresenter
       x:Name="ButtonContentPresenter"
       HorizontalAlignment="Left"
       VerticalAlignment="Stretch"
       Margin="{TemplateBinding Padding}"
       Content="{TemplateBinding Content}"
       Grid.Column="0"
       />
      <Border 
       x:Name="PART_DownArrowBorder"
       HorizontalAlignment="Stretch"
       VerticalAlignment="Stretch"
       Background="Transparent"
       Focusable="False"
       Grid.Column="1"
       >
       <Path 
        Focusable="False"
        x:Name="PART_DownArrow"
        Data="M 3,0 L 6.5,4 L 10,0" 
        Width="12" 
        HorizontalAlignment="Right"
        VerticalAlignment="Center"
        Fill="Black" SnapsToDevicePixels="True"
        >
       </Path>
      </Border>
     </Grid>
    </ToggleButton>
   </ControlTemplate>
  </Setter.Value>
 </Setter>
</Style>