UWP如何让主题资源响应RequestedTheme

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.ThemeDictionaries>
            <ResourceDictionary x:Key="Light">
                <!-- 定义浅色主题资源 -->
            </ResourceDictionary>
            <ResourceDictionary x:Key="Dark">
                <!-- 定义深色主题资源 -->
            </ResourceDictionary>
        </ResourceDictionary.ThemeDictionaries>
    </ResourceDictionary>
</Application.Resources>

<TextBlock Text="Hello World!" Foreground="{DynamicResource MyTextBrush}" />

if (Application.Current.RequestedTheme == ApplicationTheme.Light)
{
    // 切换到浅色主题
    Application.Current.Resources.ThemeDictionaries["Light"] =
        new ResourceDictionary { Source = new Uri("ms-appx:///LightTheme.xaml") };
}
else
{
    // 切换到深色主题
    Application.Current.Resources.ThemeDictionaries["Dark"] =
        new ResourceDictionary { Source = new Uri("ms-appx:///DarkTheme.xaml") };
}