WPF TextBox 禁用，背景色属性触发不生效问题（显示白色）

做公司项目时，发现Text在trigger中设置禁用颜色无效，一直显示白色，随后改成在模板中设置就可以了。

如下：

<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Border Name="Bd" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" SnapsToDevicePixels="true">
<ScrollViewer Name="PART_ContentHost" Background="{TemplateBinding Background}" Foreground="{TemplateBinding Foreground}" FontSize="{TemplateBinding FontSize}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" FontFamily="{TemplateBinding FontFamily}" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Background" Value="Gray"/>
<Setter TargetName="PART_ContentHost" Property="Background" Value="Gray"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>