wpf 触摸屏 button 背景为null的 问题

<!-- button样式-->

            <Style x:Key="myBtn" TargetType="Button" >

                <!--<Setter Property="Background" Value="{x:Null}" />-->

                <Setter Property="Background" Value="Transparent" />

                <Setter Property="BorderBrush" Value="White"/>

                <Setter Property="BorderThickness" Value="2"/>

                <Setter Property="Foreground" Value="White"/>

                <Setter Property="Template">

                    <Setter.Value>

                        <ControlTemplate TargetType="{x:Type Button}" >

                            <Border  Name="btnBorder" BorderBrush="{TemplateBinding Control.BorderBrush}"

                                BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="12"

                                 Background="{TemplateBinding Control.Background}" >

                                <ContentPresenter Content="{TemplateBinding ContentControl.Content}" HorizontalAlignment="Center" VerticalAlignment="Center" />

                            </Border>

                        </ControlTemplate>

                    </Setter.Value>

                </Setter>

                <Style.Triggers>

                    <Trigger Property="Control.IsMouseOver" Value="true">

                        <Setter Property="Control.Background" Value="#3c4d70" />

                        <Setter Property="BorderThickness" Value="0"/>

                    </Trigger>

                    <Trigger Property="Control.IsFocused" Value="true">

                        <Setter Property="Control.Background" Value="#3c4d70" />

                        <Setter Property="BorderThickness" Value="0"/>

                    </Trigger>

                </Style.Triggers>

            </Style>