WPF DatePicker内边框停用

<Style TargetType="DatePicker">
    <Setter Property="BorderThickness" Value="0" />
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="DatePicker">
                <Grid>
                    <Border Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}" />
                    <TextBox x:Name="PART_TextBox"
                             BorderThickness="0"
                             Background="Transparent"
                             IsReadOnly="True"
                             Text="{Binding Path=SelectedDate, RelativeSource={RelativeSource TemplatedParent}, StringFormat='yyyy-MM-dd', Mode=TwoWay}" />
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>