Extensions.cs

static class Extensions {

		#if __IOS__
		public static Font ToFont (this UIKit.UIFont font)
		{
			return Font.OfSize (font.Name, font.PointSize);
		}
		#endif

}

Fonts.cs

public static class Fonts {

        public static Font LargeTitle =>
        #if __IOS__
            UIKit.UIFont.GetPreferredFontForTextStyle (UIKit.UIFontTextStyle.LargeTitle).ToFont ();
        #else
            Font.SystemFontOfSize (NamedSize.Large, FontAttributes.Bold); 
        #endif

        public static Font ThinBody =>
        #if __IOS__
            UIKit.UIFontMetrics.DefaultMetrics.GetScaledFont (UIKit.UIFont.SystemFontOfSize (18, UIKit.UIFontWeight.UltraLight)).ToFont ();
        #else
            Font.Default;
        #endif
}