让我向您展示一种使用VisualTreeHelper的方法。
首先，您需要有一种通过VisualTree搜索控件的方法。

public static IEnumerable<T> FindChildren<T>(DependencyObject parent)
    where T : DependencyObject
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(parent, i);

        if (child is T childAsT)
        {
            yield return childAsT;
        }

        foreach (T grandChild in FindChildren<T>(child))
        {
            yield return grandChild;
        }
    }
}

然后，在DatePicker的CalendarOpened事件处理程序中，我们可以找到Popup及其内部的控件。Popup.Child是一个Calender控件，其中包含50个TextBlock。索引为1到7的项是表示一周中的天数的TextBlocks。

private void DatePickerControl_CalendarOpened(object sender, RoutedEventArgs e)
{
    if (FindChildren<Popup>(DatePickerControl).FirstOrDefault() is not Popup popup)
    {
        return;
    }

    double fontSize = 20;

    if (FindChildren<Button>(popup.Child)
        .Where(x => x.Name == "PART_HeaderButton")
        .FirstOrDefault() is Button headerButton)
    {
        headerButton.FontSize = fontSize;
    }

    if (FindChildren<TextBlock>(popup.Child) is IEnumerable<TextBlock> textBlocks)
    {
        int targetChildIndex = 1;

        for (int i = 0; i < 7; i++)
        {
            TextBlock dayOfTheWeedTextBlock = textBlocks.ElementAt(targetChildIndex);
            dayOfTheWeedTextBlock.FontSize = fontSize;
            var text = dayOfTheWeedTextBlock.Text;
            targetChildIndex++;
        }
    }

    if (FindChildren<Grid>(popup.Child)
        .Where(x => x.Name == "PART_YearView")
        .FirstOrDefault() is Grid yearViewGrid)
    {
        foreach (CalendarButton calendarButton in FindChildren<CalendarButton>(yearViewGrid))
        {
            calendarButton.FontSize = fontSize;
        }
    }

    if (FindChildren<Button>(popup.Child)
        .Where(x => x.Name == "PART_NextButton")
        .FirstOrDefault() is Button nextButton)
    {
        nextButton.RenderTransform = new ScaleTransform(1.5, 1.5);
        nextButton.RenderTransformOrigin = new Point(0.5, 0.5);
    }

}