<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:ModernWpf.Controls">

    <Style x:Key="DefaultGroupItemStyle" TargetType="GroupItem">
        <Setter Property="OverridesDefaultStyle" Value="True" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="GroupItem">
                    <StackPanel>
                        <local:ListViewHeaderItem
                            x:Name="PART_Header"
                            Content="{TemplateBinding Content}"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"
                            ContentStringFormat="{TemplateBinding ContentStringFormat}" />
                        <ItemsPresenter x:Name="ItemsPresenter" />
                    </StackPanel>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style TargetType="GroupItem" BasedOn="{StaticResource DefaultGroupItemStyle}" />

</ResourceDictionary>