假设这个ComboBox位于MainPage.xaml中。

<ComboBox x:Name="themeMode" SelectionChanged="Themes_SelectionChanged">
    <ComboBoxItem>Light</ComboBoxItem>
    <ComboBoxItem>Dark</ComboBoxItem>
    <ComboBoxItem>Defaut</ComboBoxItem>                    
</ComboBox>

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然后在MainPage.xaml.cs中

private void Themes_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (sender is ComboBox comboBox &&
        comboBox.SelectedItem is ComboBoxItem comboBoxItem &&
        comboBoxItem.Content is string themeString &&
        Enum.TryParse(themeString, out ElementTheme theme) is true)
    {
        this.RequestedTheme = theme;
    }
}

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此外，代替在XAML中手动添加项，您可以通过这种方式完成。

MainPage.xaml中：

public ImmutableArray<string> ElementThemeOptions { get; } = ImmutableArray.Create(Enum.GetNames<ElementTheme>());

private void Themes_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (sender is ComboBox comboBox &&
        comboBox.SelectedValue is string themeString &&
        Enum.TryParse(themeString, out ElementTheme theme) is true)
    {
        this.RequestedTheme = theme;
    }
}

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在MainPage.xaml中：

<ComboBox
    ItemsSource="{x:Bind ElementThemeOptions}"
    SelectionChanged="Themes_SelectionChanged" />

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更新

不要忘记在根页面中执行此操作，在本例中为MainPage。

<Window
    ...
    ...>
    <local:MainPage />
</Window>

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