Как удалить белую область в Datagrid WPF

Короткий путь

В вашем объекте DataGrid измените свойство HeadersVisibility на столбец

<DataGrid HeadersVisibility = "Column" ... \>

Долгий путь (больше стиля)

Сосредоточьтесь на DataGridRowHeader, удалите линию, чтобы она исчезла, измените свойство Background, чтобы сделать ее черной или нужного вам цвета.

<Style x:Key = "DataGridRowStyle1" TargetType = "{x:Type DataGridRow}">
    <Setter Property = "Background" Value = "{DynamicResource {x:Static Colors.Red}}"/>
    <Setter Property = "SnapsToDevicePixels" Value = "true"/>
    <Setter Property = "Validation.ErrorTemplate" Value = "{x:Null}"/>
    <Setter Property = "ValidationErrorTemplate">
        <Setter.Value>
            <ControlTemplate>
                <TextBlock Foreground = "Red" Margin = "2,0,0,0" Text = "!" VerticalAlignment = "Center"/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property = "Template">
        <Setter.Value>
            <ControlTemplate TargetType = "{x:Type DataGridRow}">
                <Border x:Name = "DGR_Border" Background = "{TemplateBinding Background}" BorderThickness = "{TemplateBinding BorderThickness}" BorderBrush = "{TemplateBinding BorderBrush}" SnapsToDevicePixels = "True">
                    <SelectiveScrollingGrid>
                        <SelectiveScrollingGrid.ColumnDefinitions>
                            <ColumnDefinition Width = "Auto"/>
                            <ColumnDefinition Width = "*"/>
                        </SelectiveScrollingGrid.ColumnDefinitions>
                        <SelectiveScrollingGrid.RowDefinitions>
                            <RowDefinition Height = "*"/>
                            <RowDefinition Height = "Auto"/>
                        </SelectiveScrollingGrid.RowDefinitions>
                        <DataGridCellsPresenter Grid.Column = "1" ItemsPanel = "{TemplateBinding ItemsPanel}" SnapsToDevicePixels = "{TemplateBinding SnapsToDevicePixels}"/>
                        <DataGridDetailsPresenter Grid.Column = "1" Grid.Row = "1" SelectiveScrollingGrid.SelectiveScrollingOrientation = "{Binding AreRowDetailsFrozen, Converter = {x:Static DataGrid.RowDetailsScrollingConverter}, ConverterParameter = {x:Static SelectiveScrollingOrientation.Vertical}, RelativeSource = {RelativeSource AncestorType = {x:Type DataGrid}}}" Visibility = "{TemplateBinding DetailsVisibility}"/>
                        <!--HERE: Change the background color property for the color, Or remove the whole line to make it disapper-->
                        <DataGridRowHeader Grid.RowSpan = "2" Background = "Black" SelectiveScrollingGrid.SelectiveScrollingOrientation = "Vertical" Visibility = "{Binding HeadersVisibility, Converter = {x:Static DataGrid.HeadersVisibilityConverter}, ConverterParameter = {x:Static DataGridHeadersVisibility.Row}, RelativeSource = {RelativeSource AncestorType = {x:Type DataGrid}}}"/>
                    </SelectiveScrollingGrid>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <Trigger Property = "IsNewItem" Value = "True">
            <Setter Property = "Margin" Value = "{Binding NewItemMargin, RelativeSource = {RelativeSource AncestorType = {x:Type DataGrid}}}"/>
        </Trigger>
    </Style.Triggers>
</Style>