<DatePicker Name="picker" Grid.Column="5" Grid.Row="0" SelectedDate="{Binding Patient.Birthday, StringFormat=yyyy-MM-dd, Converter={StaticResource DateConverter}}" VerticalAlignment="Center" SelectedDateChanged="DatePicker_SelectedDateChanged" BorderThickness="0" Style="{DynamicResource DatePickerStyle1}
 
">
            <DatePicker.Resources>
                <Style TargetType="DatePickerTextBox">
                    <Setter Property="BorderThickness" Value="0"/>
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate>
                                <TextBox x:Name="
 
PART_TextBox" Text="{Binding Path=SelectedDate, StringFormat={}{0:yyyy-MM-dd}, RelativeSource={RelativeSource AncestorType={x:Type DatePicker}}}" />
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </DatePicker.Resources>
        </DatePicker>

Style="{DynamicResource DatePickerStyle1}"，標紅的樣式， 是不存在的。

試了半天，沒成功，無意中碰到了這個bug？哈哈