How to set border thickness for the element of WPF DockingManager?

<Window x:Class="NewText_WPfapplication.MainWindow"
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:syncfusion="http://schemas.syncfusion.com/wpf"
                 Title="MainWindow" Height="350" Width="525">
       <Grid x:Name="Grid1">
            <syncfusion:DockingManager x:Name="_DockingNew" ElementBorderThickness="5">
                <ContentControl syncfusion:DockingManager.Header="Child1" syncfusion:DockingManager.State="Dock"/>
                <ContentControl syncfusion:DockingManager.Header="Child2" syncfusion:DockingManager.State="Dock"/>
                <ContentControl syncfusion:DockingManager.Header="Child3" syncfusion:DockingManager.State="Dock"/>
                <ContentControl syncfusion:DockingManager.Header="Child4" syncfusion:DockingManager.State="Dock"/>
                <ContentControl syncfusion:DockingManager.Header="Child5" syncfusion:DockingManager.State="Dock"/>
             </syncfusion:DockingManager>
        </Grid>
</Window>

using Syncfusion.Windows.Tools.Controls;
using Syncfusion.Windows.Shared;
namespace NewText_WPfapplication
{
   /// <summary>
   /// Interaction logic for MainWindow.xaml
   /// </summary>
   public partial class MainWindow : Window
   {
      public MainWindow()
      {
         InitializeComponent();
         DockingManager _Dockingnew = new DockingManager();
         _Dockingnew.ElementBorderThickness = new Thickness(5);
         ContentControl content = new ContentControl();
         DockingManager.SetHeader(content, "Child1");
         DockingManager.SetState(content, DockState.Dock);
         ContentControl content1 = new ContentControl();
         DockingManager.SetHeader(content1, "Child2");
         DockingManager.SetState(content1, DockState.Dock);
         ContentControl content2 = new ContentControl();
         DockingManager.SetHeader(content2, "Child3");
         DockingManager.SetState(content2, DockState.Dock);
         ContentControl content3 = new ContentControl();
         DockingManager.SetHeader(content3, "Child4");
         DockingManager.SetState(content3, DockState.Dock);
         ContentControl content4 = new ContentControl();
         DockingManager.SetHeader(content4, "Child5");
         DockingManager.SetState(content4, DockState.Dock);
         _Dockingnew.Children.Add(content);
         _Dockingnew.Children.Add(content1);
         _Dockingnew.Children.Add(content2);
         _Dockingnew.Children.Add(content3);
         _Dockingnew.Children.Add(content4);
         Grid1.Children.Add(_Dockingnew);
      }
   }
}