How to style a DatePicker ?

Question

I downloaded a WPFToolkit source, because I wanted to override the default generic template of a DatePicker. For example, I wanted to override this TextBox:

<primitives:DatePickerTextBox x:Name="PART_TextBox" 
            Grid.Row="0" Grid.Column="0" 
            Foreground="{TemplateBinding Foreground}"                  
            HorizontalContentAlignment="Stretch"
            VerticalContentAlignment="Stretch" />

That means that writing this in my project :

<Style TargetType="{x:Type toolkit:DatePickerTextBox}">
                <Setter Property="Text" Value="Bitte wählen" />
                <Setter Property="MinHeight" Value="20" />

works perfectly. But what if I want to change VerticalContentAlignment="Stretch" to VerticalContentAlignment="Center" ?? The default style always overrides it. Thanks for response!