I have implemented everything I wanted including putting borders around the text, changing the size of the font and changing color of the text but for some reason applying a different font doesn't seem to be working.

I am loading the following in MauiProgram.cs

.ConfigureFonts(fonts =>

fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
fonts.AddFont("GothicA1-Medium.ttf", "GothicA1Medium");
fonts.AddFont("GothicA1-SemiBold.ttf", "GothicA1Semibold");
fonts.AddFont("GothicA1-Regular.ttf", "GothicA1Regular");
fonts.AddFont("GothicA1-Thin.ttf", "GothicA1Thin");
fonts.AddFont("DancingScript-Regular.ttf", "DancingScriptRegular");
fonts.AddFont("fa-solid-900.ttf", "FAS");
And I am extending ShellTabLayoutAppearanceTracker, 
public class CustomTabLayoutAppearance(IShellContext shellContext) : ShellTabLayoutAppearanceTracker(shellContext)
public override void SetAppearance(TabLayout tabLayout, ShellAppearance appearance)
    base.SetAppearance(tabLayout, appearance);
// Accessing and customizing tab items
var fontFamily = Typeface.Create("GothicA1Regular", TypefaceStyle.Normal);
fontFamily = Typeface.Create("DancingScript", TypefaceStyle.Normal);
for (int i = 0; i < tabLayout.TabCount; i++)
    var tab = tabLayout.GetTabAt(i);
    var tabView = tab.View as LinearLayout;
    if (tabView is null)
        return;
    var textView = tabView?.GetChildAt(1) as TextView;
    tabView.SetBackgroundColor(Android.Graphics.Color.White);
    var customTextView = new TextView(tabLayout.Context);
    customTextView.Text = tab.Text;            
    customTextView.TransformationMethod = null;
    // Set your desired font size here
    if (tab.IsSelected)
        customTextView.SetTextColor(Android.Graphics.Color.White); // Example: Change text color
        customTextView.SetBackgroundColor(Android.Graphics.Color.White);
        ApplyBackgroundAndBorderToTextView(customTextView, true);
        //customTextView.Visibility = Android.Views.ViewStates.Gone;
        customTextView.SetTextColor(Android.Graphics.Color.Black);
        ApplyBackgroundAndBorderToTextView(customTextView, false);
    customTextView.SetTypeface(fontFamily, TypefaceStyle.Normal);
    // Set font size
    customTextView.SetTextSize(Android.Util.ComplexUnitType.Sp, 15);
    customTextView.LetterSpacing = (float)0.01;
    tabView.RemoveAllViews();
    tabView.AddView(customTextView);
    customTextView.Invalidate();
all the custom methods within this override method works. I have tried both GoathicA1Regular and DancingScript that's why I have two lines at the start of the method.
Could you advise me how I can fix this?
Thank you very much in advance.
											
Firstly, did you set ConfigureMauiHandlers for shell custom render in the MauiProgram.cs like following code
  builder
                .UseMauiApp<App>()
                .ConfigureMauiHandlers(handlers => {
#if ANDROID
                    handlers.AddHandler(typeof(Shell), typeof(CustomShellRenderer));
#endif
If you change the color to red tabView.SetBackgroundColor(Android.Graphics.Color.Red);, will you get the correct background color?
											
Hi Leon
Thanks for your reply. Yes I have the handler code that you have specified in mauiprogram.cs file.
As I have mentioned, everything works, including the background colour, I am even adding a border around the textview which works perfectly as expected, even the font size specification works correctly. However applying the different font doesn't want to play nicely :(
I initially had issues with the font size so instead of using the existing textview of the tabview, I am creating a new one and adding it to the tabview.
Please advise if you have anything I can try. 
			 
Hello,
Please create a font folder in the Platforms/Android/Resources folder.
Then copy your font files in font folder. Please change your font file's name with Lowercase characters and _ like alexbrush_regular.ttf, make sure this font's build action is AndroidResource
Then, you can set the font with textView.SetTypeface(Platform.CurrentActivity.Resources.GetFont(Resource.Font.alexbrush_regular),TypefaceStyle.Normal); in the custom renderer like following code.
public void SetAppearance(TabLayout tabLayout, ShellAppearance appearance)
      for (int i = 0; i < tabLayout.TabCount; i++)
          var tab = tabLayout.GetTabAt(i);
          var tabView = tab.View as LinearLayout;
          if (tabView is null)
              return;
          var textView = tabView?.GetChildAt(1) as TextView;
          textView.SetTypeface(Platform.CurrentActivity.Resources.GetFont(Resource.Font.alexbrush_regular),TypefaceStyle.Normal);
Best Regards,
Leon Lu
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Thank you very much Leon. I have got this working.
I presume I have to apply something totally different for iOS. 
Have a great day!.