Style datagrid table - Top left corner

Question

I am styling a datatable but I can't figure out how to style the top left filed of the datagrid. It is the gray field in this picture:

Do you know how to do it?

Here is my style so far:

<Style TargetType="{x:Type DataGrid}">
    <Setter Property="Margin" Value="5" />
    <Setter Property="Background">
        <Setter.Value>
            <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                <GradientStop Color="White"/>
                <GradientStop Color="AliceBlue" Offset="1"/>
            </LinearGradientBrush>
        </Setter.Value>
    </Setter>
    <Setter Property="RowBackground">
        <Setter.Value>
            <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                <GradientStop Color="#BAF0FF"/>
                <GradientStop Color="PowderBlue" Offset="1"/>
            </LinearGradientBrush>
        </Setter.Value>
    </Setter>
    <Setter Property="AlternatingRowBackground">
        <Setter.Value>
            <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                <GradientStop Color="White"/>
                <GradientStop Color="AliceBlue" Offset="1"/>
            </LinearGradientBrush>
        </Setter.Value>
    </Setter>
    <Setter Property="HorizontalGridLinesBrush" Value="LightGray" />
    <Setter Property="VerticalGridLinesBrush" Value="LightGray" />
</Style>

Answer 1

From this answer I was able to create this code which correctly sets the style of the button:

<DataGrid>    
    <DataGrid.Resources>
        <Style TargetType="Button" x:Key="{ComponentResourceKey ResourceId=DataGridSelectAllButtonStyle, TypeInTargetAssembly={x:Type DataGrid}}">
            <Setter Property="Background" Value="Black" />
        </Style>
    </DataGrid.Resources>
</DataGrid>