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>