public FluentElement <T> Insert(int index, VisualElement element)
        {
            FluentUITK.AddCoreStyles(element);

            x.Insert(index, element);
            onAddChild?.Invoke(element);

            return(x);
        }
 internal FluentElement <T> AddCoreStyles()
 {
     FluentUITK.AddCoreStyles(x); return(x);
 }