How to remove border of header cell

[Singla]

Hi

I'm not able to remove the borders of the header cell . Even i have given BorderThickness zero. please find attacehd screenshot to see the problem. There is white border in between the cell. how to remove this border?

<LinearGradientBrush x:Name="GridView_HeaderCellBackground" EndPoint="0.5,1" StartPoint="0.5,0" MappingMode="RelativeToBoundingBox" >
          <GradientStop Color="#7E000000"/>
          <GradientStop Color="Black" Offset="0.3"/>
          <GradientStop Color="Black" Offset="0.852"/>
          <GradientStop Color="#80000000" Offset="1"/>
      </LinearGradientBrush>
      <Style TargetType="telerik:GridViewHeaderCell">
          <Setter Property="Foreground" Value="White"/>
          <Setter Property="Background" Value="{StaticResource GridView_HeaderCellBackground}"/>
          <Setter Property="BorderBrush" Value="Black"/>
          <Setter Property="BorderThickness" Value="10"/>
      </Style>