How to change Expander Header background dynamicliy?

Question

I have some Expander in my Window... I want when my Expander isExpanded its header background changed... How to I do this?

Answer 1

<Style x:Key="ExpanderStyle" TargetType="{x:Type Expander}">
    <Style.Triggers>
        <Trigger Property="IsExpanded" Value="True">
            <Setter Property="HeaderTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <Border Background="Green">
                            <ContentControl Content="{Binding}"/>
                        </Border>
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </Trigger>
        <Trigger Property="IsExpanded" Value="False">
            <Setter Property="HeaderTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <Border Background="Red">
                            <ContentControl Content="{Binding}"/>
                        </Border>
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </Trigger>
    </Style.Triggers>
</Style>