Style for dual header groupbox

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:ctl="clr-namespace:Velib.Controls"
                    xmlns:conv="clr-namespace:Velib.Converters">

    <Style TargetType="{x:Type ctl:DualHeaderGroupBox}">

        <Setter Property="BorderBrush" Value="#D5DFE5" />
        <Setter Property="BorderThickness" Value="1" />

        <Setter Property="Template">
            <Setter.Value>

                <ControlTemplate TargetType="{x:Type ctl:DualHeaderGroupBox}">
                    <ControlTemplate.Resources>
                        <conv:DualBorderGapMaskConverter x:Key="DualBorderGapMaskConverter" />
                    </ControlTemplate.Resources>

                    <Grid SnapsToDevicePixels="True">

                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="*"/>
                            <RowDefinition Height="6"/>
                        </Grid.RowDefinitions>

                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="6"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="6"/>
                        </Grid.ColumnDefinitions>

                        <Border Grid.Row="1" Grid.Column="0"
                                Grid.RowSpan="3" Grid.ColumnSpan="5"
                                CornerRadius="4"
                                Background="{TemplateBinding Background}"
                                BorderBrush="Transparent"
                                BorderThickness="{TemplateBinding BorderThickness}"/>

                        <Border Name="LeftHeader"
                                Grid.Row="0" Grid.Column="1"
                                Grid.RowSpan="2"
                                HorizontalAlignment="Left"
                                Padding="3,1,3,0">
                            <ContentPresenter ContentSource="LeftHeader"
                                              RecognizesAccessKey="true"
                                              SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                        </Border>

                        <Border Name="RightHeader"
                                Grid.Row="0" Grid.Column="3"
                                Grid.RowSpan="2"
                                HorizontalAlignment="Right"
                                Padding="3,1,3,0">
                            <ContentPresenter ContentSource="RightHeader"
                                              RecognizesAccessKey="true"
                                              SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                        </Border>

                        <ContentPresenter Grid.Row="2" Grid.Column="1"
                                          Grid.ColumnSpan="3"
                                          Margin="{TemplateBinding Padding}"
                                          SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />

                        <Border Grid.Row="1"
                                Grid.RowSpan="3" Grid.ColumnSpan="5"
                                CornerRadius="4"
                                BorderThickness="{TemplateBinding BorderThickness}"
                                BorderBrush="White"
                                IsHitTestVisible="False">
                            <Border.OpacityMask>
                                <MultiBinding Converter="{StaticResource DualBorderGapMaskConverter}" ConverterParameter="7">
                                    <Binding ElementName="LeftHeader" Path="ActualWidth" />
                                    <Binding ElementName="RightHeader" Path="ActualWidth" />
                                    <Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}" />
                                    <Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}" />
                                </MultiBinding>
                            </Border.OpacityMask>
                            <Border BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="3">
                                <Border BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="White" CornerRadius="2" />
                            </Border>
                        </Border>

                    </Grid>

                </ControlTemplate>

            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>