Try to use below given style for DatePickerTextBox instead of DatePicker.
<Style TargetType="{x:Type DatePickerTextBox}">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<TextBox BorderThickness="0" Style="{x:Null}" x:Name="PART_TextBox"
Text="{Binding Path=SelectedDate, StringFormat='dd-MMM-yyyy',
RelativeSource={RelativeSource AncestorType={x:Type DatePicker}}}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Hope this helps.