此弹出按钮是一个DatePickerFlyoutPresenter，从generic.xaml中，您可以了解到这是一个名为 HighlightRect 的Grid。
要更改 HighlightRect 的Background：

public MainPage()
{
    this.InitializeComponent();
    this.DatePickerControl.LayoutUpdated += DatePickerControl_LayoutUpdated;
}

private void DatePickerControl_LayoutUpdated(object? sender, object e)
{
    if (VisualTreeHelper
        .GetOpenPopupsForXamlRoot(this.XamlRoot)
        .FirstOrDefault() is Popup popup &&
        popup.Child.FindDescendant<Grid>(x => x.Name is "HighlightRect") is Grid highlightRect &&
        popup.Child.FindDescendants().OfType<LoopingSelector>() is IEnumerable<LoopingSelector> loopingSelectors)
    {
        highlightRect.Background = new SolidColorBrush(Colors.LightGreen);

        foreach (LoopingSelector loopingSelector in loopingSelectors)
        {
            loopingSelector.FontSize = 20;
        }
    }
}

备注：

FindDescendant来自CommunityToolkit.WinUI.Extensions NuGet包。