DataGridColumnHeaderStyle背景色未填充完整的标题

Question

我想在xaml中设置DataGrid的背景颜色。我创建了以下样式：

<Style x:Key="StandardSpaltenKopf" TargetType="{x:Type DataGridColumnHeader}">
    <Setter Property="Background" Value="#303433" />
    <Setter Property="Foreground" Value="White"/>
    <Setter Property="FontFamily" Value="Segoe UI"/>
    <Setter Property="FontSize" Value="14"/>
    <Setter Property="FontWeight" Value="Bold"/>
    <Setter Property="Height" Value="20"/>
    <Setter Property="VerticalAlignment" Value="Center"/>
    <Setter Property="HorizontalAlignment" Value="Center"/>
    <Setter Property="Margin" Value="0"/>
</Style>

我设置的背景色仅在文本后面，而列标题的其余部分以另一种颜色显示（请参见图片中的April和Mai）。如何填充其余的列标题？

更新1：

以下内容也不起作用：

<Style x:Key="StandardSpaltenKopf" TargetType="{x:Type DataGridColumnHeader}">
    <Setter Property="Foreground" Value="White"/> 
    <Setter Property="FontFamily" Value="Segoe UI"/>
    <Setter Property="FontSize" Value="16"/>
    <Setter Property="FontWeight" Value="Bold"/>
    <Setter Property="Height" Value="28"/>
    <Setter Property="VerticalAlignment" Value="Center"/>
    <Setter Property="HorizontalAlignment" Value="Center"/>
    <Setter Property="Margin" Value="0"/>
    <Setter Property="BorderBrush" Value="{StaticResource Dunkelgrau}" />
    <Setter Property="Padding" Value="0" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
                <Grid Background="{StaticResource Dunkelgrau}" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
                    <ContentPresenter></ContentPresenter>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>