How to disable white vertical line in the datagrid header row?

[CSHubbardCF]

I've spent hours at this point looking for the property that is creating this vertical white line.

I've searched all over "https://github.com/MahApps/MahApps.Metro/blob/develop/src/MahApps.Metro/Styles/Controls.DataGrid.xaml"
trying to find the correct property to overwrite in my custom resource dictionary to remove this vertical line and have had absolutely no luck.

It is definitely not the border as I've made that large and green in the picture for emphasis sake. Ive tried "VerticalGridLinesBrush", as well as "Separator". Different setters in both the DataGridColumnHeader as well as the DataGrid and
DataGridRowHeader. At this point I have a feeling its buried somewhere in the "template" of DataGridColumnHeader but I thought to as here before diving into that.

Thank you for any help you can provide.

[CSHubbardCF]

I found the solution after looking into the template. It turns out that the setter that was creating the vertical line was "MahApps.Styles.Thumb.ColumnHeaderGripper" being used inside the "DataGridColumnHeader"'s "Template" property. I imported both the "Template" tag into my override ResourceDictionary as well as the MahApps.Styles.Thumb.ColumnHeaderGripper which I renamed to "CustomColumnHeaderGripper" and customized.

<!--  DataGridColumnHeader style  -->
<Style
    x:Key="ColumnHeaderStyle1"
    BasedOn="{StaticResource MahApps.Styles.DataGridColumnHeader}"
    TargetType="{x:Type DataGridColumnHeader}">
    <Setter Property="Height" Value="60" />
    <Setter Property="Padding" Value="10,10,10,10" />
    <Setter Property="FontSize" Value="16" />
    <Setter Property="FontWeight" Value="Bold" />
    <Setter Property="BorderThickness" Value="0,0,3,0" />
    <Setter Property="BorderBrush" Value="{StaticResource DefaultBackgroundColor}" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="Auto" />
                    </Grid.ColumnDefinitions>
                    <Border
                        x:Name="BackgroundBorder"
                        Grid.ColumnSpan="2"
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}" />
                    <mah:ContentControlEx
                        x:Name="HeaderContent"
                        Grid.Column="0"
                        Margin="{TemplateBinding BorderThickness}"
                        Padding="{TemplateBinding Padding}"
                        HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                        VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                        Content="{TemplateBinding Content}"
                        ContentCharacterCasing="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(mah:ControlsHelper.ContentCharacterCasing)}"
                        ContentStringFormat="{TemplateBinding ContentStringFormat}"
                        ContentTemplate="{TemplateBinding ContentTemplate}"
                        ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"
                        RecognizesAccessKey="{TemplateBinding mah:ControlsHelper.RecognizesAccessKey}"
                        SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                    <Path
                        x:Name="SortArrow"
                        Grid.Column="1"
                        Width="8"
                        Height="10"
                        Margin="0,0,8,2"
                        VerticalAlignment="Center"
                        Fill="{DynamicResource MahApps.Brushes.Gray2}"
                        RenderTransformOrigin="0.5,0.5"
                        Stretch="Fill"
                        Visibility="Collapsed" />
                    <Thumb
                        x:Name="PART_LeftHeaderGripper"
                        HorizontalAlignment="Left"
                        Background="Transparent"
                        Style="{DynamicResource CustomColumnHeaderGripper}" />
                    <Thumb
                        x:Name="PART_RightHeaderGripper"
                        Grid.Column="1"
                        HorizontalAlignment="Right"
                        Style="{DynamicResource CustomColumnHeaderGripper}" />
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="SortDirection" Value="Ascending">
                        <Setter TargetName="SortArrow" Property="Data" Value="F1 M 34,57L 42,57L 42,32.25L 52,42.25L 52,31.75L 38,17.75L 24,31.75L 24,42.25L 34,32.25L 34,57 Z " />
                        <Setter TargetName="SortArrow" Property="Visibility" Value="Visible" />
                    </Trigger>
                    <Trigger Property="SortDirection" Value="Descending">
                        <Setter TargetName="SortArrow" Property="Data" Value="F1 M 42,19.0002L 34,19.0002L 34,43.7502L 24,33.7502L 24,44.2502L 38,58.2502L 52,44.2502L 52,33.7502L 42,43.7502L 42,19.0002 Z " />
                        <Setter TargetName="SortArrow" Property="Margin" Value="0,0,8,0" />
                        <Setter TargetName="SortArrow" Property="Visibility" Value="Visible" />
                    </Trigger>
                    <Trigger Property="DisplayIndex" Value="0">
                        <Setter TargetName="PART_LeftHeaderGripper" Property="Visibility" Value="Collapsed" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<Style x:Key="CustomColumnHeaderGripper" TargetType="{x:Type Thumb}">
    <Setter Property="Background" Value="{StaticResource DefaultBackgroundColor}" />
    <Setter Property="Cursor" Value="SizeWE" />
    <Setter Property="SnapsToDevicePixels" Value="True" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Thumb}">
                <Border
                    Margin="{TemplateBinding Padding}"
                    Background="Transparent"
                    BorderBrush="{TemplateBinding Background}"
                    BorderThickness="0,0,1,0"
                    SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="Width" Value="8" />
</Style>