检查您的App.xaml以包含<XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />资源字典，如下所示：

<Application
  x:Class="Your.Project.App"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Application.Resources>
    <ResourceDictionary>
       <ResourceDictionary.MergedDictionaries>
         <!-- the following line is important -->
         <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
      </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
  </Application.Resources>
</Application>