// ...
// Inside the .designer.cs file of your form
this.radTabbedFormControl1 = new MyRadTabbedFormControl();
//...

public class MyRadTabbedFormControl : RadTabbedFormControl
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadTabbedFormControl).FullName;
        }
    }

    protected override void CreateChildItems(RadElement parent)
    { 
        RadTabbedFormControlElement mainElement = new MyRadTabbedFormControlElement();

        typeof(RadTabbedFormControl).GetField("tabbedFormControlElement", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(this, mainElement);
        typeof(RadTabbedFormControlElement).GetField("owner", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(mainElement, this);

        this.RootElement.Children.Add(mainElement);

        this.RootElement.Name = this.Name;
    }
}

public class MyRadTabbedFormControlElement : RadTabbedFormControlElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadTabbedFormControlElement);
        }
    }

    protected override void UpdateTabBounds(RadTabbedFormControlTab tab)
    {
        if (this.ElementState != ElementState.Loaded)
        {
            return;
        }

        Rectangle contentAreaBoundingRect = this.GetContentAreaRectangle();
        Rectangle contentRect = this.OffsetTabContent(contentAreaBoundingRect);

        if (contentRect.Width <= 0 || contentRect.Height <= 0)
        {
            return;
        }

        base.UpdateTabBounds(tab);
    }
}