要更改NavigationBar中的标题字体，请阅读此文档，在每个contentPage中自定义Shell.TitleView：

<Shell.TitleView>
    <Label Text="customTitle" FontSize="30"/>
</Shell.TitleView>

字符串
要更改选项卡栏标题字体，您需要一个自定义渲染器：

[assembly: ExportRenderer(typeof(AppShell), typeof(MyShellRenderer))]
namespace App30.Droid
{
    public class MyShellRenderer : ShellRenderer
    {
        public MyShellRenderer(Context context) : base(context)
        {
        }

        protected override IShellBottomNavViewAppearanceTracker CreateBottomNavViewAppearanceTracker(ShellItem shellItem)
        {
            return new CustomBottomNavAppearance();
        }
    }

    public class CustomBottomNavAppearance : IShellBottomNavViewAppearanceTracker
    {
        public void Dispose()
        {

        }

        public void ResetAppearance(BottomNavigationView bottomView)
        {

        }

        public void SetAppearance(BottomNavigationView bottomView, ShellAppearance appearance)
        {

            IMenu menu = bottomView.Menu;
            for (int i = 0; i < bottomView.Menu.Size(); i++)
            {
                IMenuItem menuItem = menu.GetItem(i);
                var title = menuItem.TitleFormatted;
                SpannableStringBuilder sb = new SpannableStringBuilder(title);

                int a = sb.Length();
                
                //here I set fontsize 20
                sb.SetSpan(new AbsoluteSizeSpan(20,true), 0, a, SpanTypes.ExclusiveExclusive);

                menuItem.SetTitle(sb);
            }

        }
    }
}

型
结果如下：

的数据
一个sample project已经上传到这里，你可以检查它。