Android:

You could use the CreateBottomNavViewAppearanceTracker to change something for the bottomview.

  [assembly: ExportRenderer(typeof(AppShell), typeof(ShellCustomRenderer))]
namespace ShellDemo.Droid
{
class ShellCustomRenderer : ShellRenderer
{

    public ShellCustomRenderer(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, IShellAppearanceElement appearance)
    {
        //put your code here
       
    }
}
}

• Shadow: Create a Shadow.xml in Drawable of Android.

  <shape 
   xmlns:android="http://schemas.android.com/apk/res/android">
  <gradient
   android:startColor="@android:color/holo_orange_light"
   android:endColor="@android:color/transparent"
   android:angle="90" />
  </shape>
  

  And then set the shadow as background of bottomview in SetAppearance.

  bottomView.SetBackgroundResource(Resource.Drawable.shadow);
  

• Border line of the bottomview. Create a Border_top.xml in Drawable of Android.

   <layer-list 
   xmlns:android="http://schemas.android.com/apk/res/android">
   <item android:top="5dip">
     <shape>
       <solid android:color="@android:color/holo_green_light"> 
       </solid>
    </shape>
   </item>
   </layer-list>
  

  And then set it as background.

  bottomView.SetBackgroundResource(Resource.Drawable.border_top);
  

iOS:

You could use the CreateTabBarAppearanceTracker to change the tabbar.

  [assembly: ExportRenderer(typeof(AppShell), typeof(ShellCustomRenderer))]
namespace ShellDemo.iOS
{
public class ShellCustomRenderer : ShellRenderer
{
    protected override IShellTabBarAppearanceTracker CreateTabBarAppearanceTracker()
    {
        return new TabBarAppearance();
    }

}

public class TabBarAppearance : IShellTabBarAppearanceTracker
{
    public void Dispose()
    {

    }

    public void ResetAppearance(UITabBarController controller)
    {
        
    }

    public void SetAppearance(UITabBarController controller, ShellAppearance appearance)
    {
    }

    public void UpdateLayout(UITabBarController controller)
    {
    }
}
}

Shadow: The iOS has a default shadow there.