Problem with Datagrid Header Background

Question

I have the following block of code setting the style of Header in my shampoo:

<Style TargetType="{x:Type DataGridColumnHeader}">
                    <Setter Property="Background" Value="DimGray"/>
                    <Setter Property="Height" Value="45"/>
                    <Setter Property="Foreground" Value="White"/>
                    <Setter Property="FontFamily" Value="Calibri"/>
                    <Setter Property="FontSize" Value="20"/>
                    <Setter Property="Padding" Value="15 0 0 0"/>
                    <Setter Property="HorizontalAlignment" Value="Stretch"/>
                </Style>

When the value of the property HorizontalAlignment="Stretch" the result is this:

But when I change the value to HorizontalAlignment="Center" happens this:

I want it to be centered but with the whole background filled.