Background in DataGrid Header - wpf

Question

I'm working on a style for my DataGrid and I have a little problem shown in the picture below.
If the header of the DataGrid doesn't fill the width There is some gradient background and a bottom and right border. Is there a way to get rid of these 2 things and make it look like in the following picture? (Picture made with PhotoShop for illustration purposes)
Below is the style for my DataGrid:
<Style TargetType="{x:Type DataGrid}">
<Style.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}"
Color="#F50099FF" />
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightTextBrushKey}"
Color="#FFCCFFFF" />
<SolidColorBrush x:Key="{x:Static SystemColors.InactiveSelectionHighlightBrushKey}"
Color="#F50099FF" />
<SolidColorBrush x:Key="{x:Static SystemColors.InactiveSelectionHighlightTextBrushKey}"
Color="#FFCCFFFF" />
</Style.Resources>
<Setter Property="IsSynchronizedWithCurrentItem"
Value="False" />
<Setter Property="IsReadOnly"
Value="True" />
<Setter Property="BorderThickness"
Value="0" />
<Setter Property="GridLinesVisibility"
Value="None" />
<Setter Property="RowHeaderWidth"
Value="0" />
<Setter Property="Background"
Value="White" />
<Setter Property="RowBackground"
Value="Transparent" />
<Setter Property="AlternatingRowBackground"
Value="#F5E5E5E5" />
<Setter Property="SelectionMode"
Value="Single" />
<Setter Property="SelectionUnit"
Value="FullRow" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type DataGrid}">
<Border BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{TemplateBinding Background}"
Padding="{TemplateBinding Padding}"
SnapsToDevicePixels="True">
<ScrollViewer x:Name="DG_ScrollViewer"
Focusable="False">
<ScrollViewer.Template>
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Button Command="ApplicationCommands.SelectAll"
Focusable="False"
Style="{DynamicResource {ComponentResourceKey ResourceId=DataGridSelectAllButtonStyle, TypeInTargetAssembly={x:Type DataGrid}}}"
Width="{Binding CellsPanelHorizontalOffset, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type DataGrid}}}">
<Button.Visibility>
<Binding Path="HeadersVisibility"
RelativeSource="{RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type DataGrid}}">
<Binding.ConverterParameter>
<DataGridHeadersVisibility>All</DataGridHeadersVisibility>
</Binding.ConverterParameter>
</Binding>
</Button.Visibility>
</Button>
<DataGridColumnHeadersPresenter x:Name="PART_ColumnHeadersPresenter"
Grid.Column="1">
<DataGridColumnHeadersPresenter.Visibility>
<Binding Path="HeadersVisibility"
RelativeSource="{RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type DataGrid}}">
<Binding.ConverterParameter>
<DataGridHeadersVisibility>Column</DataGridHeadersVisibility>
</Binding.ConverterParameter>
</Binding>
</DataGridColumnHeadersPresenter.Visibility>
</DataGridColumnHeadersPresenter>
<ScrollContentPresenter x:Name="PART_ScrollContentPresenter"
CanContentScroll="{TemplateBinding CanContentScroll}"
CanHorizontallyScroll="False"
Grid.ColumnSpan="2"
CanVerticallyScroll="False"
ContentTemplate="{TemplateBinding ContentTemplate}"
Content="{TemplateBinding Content}"
ContentStringFormat="{TemplateBinding ContentStringFormat}"
Grid.Row="1" />
<ScrollBar x:Name="PART_VerticalScrollBar"
Grid.Column="2"
Maximum="{TemplateBinding ScrollableHeight}"
Orientation="Vertical"
Grid.RowSpan="2"
Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"
Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}"
ViewportSize="{TemplateBinding ViewportHeight}" />
<ScrollBar x:Name="PART_HorizontalScrollBar"
Grid.Row="2"
Grid.ColumnSpan="2"
Maximum="{TemplateBinding ScrollableWidth}"
Orientation="Horizontal"
Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"
Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}"
ViewportSize="{TemplateBinding ViewportWidth}" />
</Grid>
</ControlTemplate>
</ScrollViewer.Template>
<ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
</ScrollViewer>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Next style is the ColumnHeader:
<Style x:Key="ColumnHeaderStyleRight"
TargetType="DataGridColumnHeader">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
<Grid>
<Border BorderBrush="#DBDBDB"
Background="White"
BorderThickness="0.5 1 1 1"
Padding="3"
CornerRadius="0 5 5 0">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="14" />
</Grid.ColumnDefinitions>
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
<Path x:Name="SortArrow"
Grid.Column="1"
HorizontalAlignment="Right"
VerticalAlignment="Center"
Width="8"
Height="6"
Margin="3 0"
Stretch="Fill"
Fill="Black"
RenderTransformOrigin="0.5 0.4"
Visibility="Collapsed"
Data="M0,0 L1,0 0.5,1 z" />
</Grid>
</Border>
<!--<Thumb x:Name="PART_LeftHeaderGripper"
HorizontalAlignment="Left"
Style="{StaticResource ColumnHeaderGripperStyle}" />
<Thumb x:Name="PART_RightHeaderGripper"
HorizontalAlignment="Right"
Style="{StaticResource ColumnHeaderGripperStyle}" />-->
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="SortDirection"
Value="Ascending">
<Setter TargetName="SortArrow"
Property="Visibility"
Value="Visible" />
<Setter TargetName="SortArrow"
Property="RenderTransform">
<Setter.Value>
<RotateTransform Angle="180" />
</Setter.Value>
</Setter>
</Trigger>
<Trigger Property="SortDirection"
Value="Descending">
<Setter TargetName="SortArrow"
Property="Visibility"
Value="Visible" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And in my actual DataGrid I have a column like this:
<DataGrid AutoGenerateColumns="False"
ItemsSource="{Binding FilteredOrders.View}"
SelectedItem="{Binding SelectedOrder}">
<DataGrid.Columns>
<DataGridTextColumn Header="Groep"
Binding="{Binding Workcenter.Section.Section}"
HeaderStyle="{StaticResource ColumnHeaderStyleRight}" />
</DataGrid.Columns>
</DataGrid>