Style expander header background color in App.xaml

Question

I'm trying to create a global style for Expander in WPF.

This is what I achieved so far:

<Style TargetType="Expander">
    <Setter Property="HeaderTemplate">
        <Setter.Value>
            <DataTemplate>
                <Border Background="Red">
                    <ContentControl Content="{Binding}" />
                </Border>
            </DataTemplate>
        </Setter.Value>
    </Setter>
</Style>

And this is how it looks:

Not very satisfying.

Question: How can I change the background color of the entire Expander header in App.xaml?