Styling issue for column header in data grid in WPF

I created a Wcf Toolkit Datagrid and below is the ColumnHeader style for it But there is still some area in the column header which is not styled as shown in the picture

http://www.freeimagehosting.net/uploads/9aba4fbd93.jpg

<Style x:Key="ColumnHeaderStyle" TargetType="{x:Type dg:DataGridColumnHeader}">
        <Setter Property="VerticalContentAlignment" Value="Center" />
        <Setter Property="Background" Value="Orange" />
        <Setter Property="Foreground" Value="White" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="dg:DataGridColumnHeader">   
                    <dg:DataGridHeaderBorder
                    x:Name="headerBorder"                      
                    Background="Orange">
                    <Border BorderThickness="2" 
                            CornerRadius="5" 
                            Background="Orange"
                            BorderBrush="DarkOrange">                            
                        <Grid>
                            <TextBlock Text="{TemplateBinding  Content}" 
                                       VerticalAlignment="Center" 
                                       HorizontalAlignment="Center" 
                                       TextWrapping="Wrap"/>                                
                            </Grid>                                
                    </Border>
                    </dg:DataGridHeaderBorder>

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

 <dg:DataGrid Grid.Row="1" Grid.RowSpan="1" 
                 Name="dgQuestion"                      
                 HorizontalAlignment="Left"
                 AutoGenerateColumns="True"
                 Width="740"
                 MinWidth="200"
                 MaxWidth="740"                         
                 Background="Wheat"
                 ColumnHeaderHeight="30"                       
                 ColumnHeaderStyle="{DynamicResource ColumnHeaderStyle}"
                 RowStyle="{StaticResource RowStyle}"
                 CanUserAddRows="False"
                 CanUserDeleteRows="False"
                 AlternationCount="2"/>