Xamarin.Forms默认导航栏字体

public class CustomNavigationPage : NavigationPage
{
    public CustomNavigationPage(Page root) : base(root)
    {
        BarBackgroundColor = Color.FromHex("#2196F3");
        BarTextColor = Color.White;
        TitleView = new StackLayout
        {
            Spacing = 0,
            Children = {
                new Label
                {
                    Text = "My App",
                    FontFamily = "Arial",
                    FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label)),
                    HorizontalTextAlignment = TextAlignment.Center,
                    TextColor = Color.White
                }
            }
        };
    }
}

public partial class App : Application
{
    public App()
    {
        InitializeComponent();

        var mainPage = new MainPage();
        var navigationPage = new CustomNavigationPage(mainPage);
        MainPage = navigationPage;
    }
}

var titleLabel = new Label
{
    Text = "My App",
    FontFamily = Device.RuntimePlatform == Device.iOS ? "HelveticaNeue-Bold" : "Arial",
    FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label)),
    HorizontalTextAlignment = TextAlignment.Center,
    TextColor = Color.White
};