how to show selected node custom treeview in wpf

Good question, because this technique is very important, because default coloring of framework elements used as tree nodes is quite ugly, so you need such code in many cases.

Let's assume you use TextBlock for your selectable items, also assume that the TextBlock has default black-on-white colors (or set all those colors programmatically when a new item is added to be in sync with selected/deselected colors use below):

lblDetailTree.SelectedItemChanged += (sender, eventArgs) => {
    TextBlock oldItem = eventArgs.OldValue as TextBlock;
    TextBlock newItem = eventArgs.NewValue as TextBlock;
    if (oldItem != null) {
        oldItem.Background = Brushes.White;
        oldItem.Foreground = Brushes.Black;
    } //if oldItem
    if (newItem != null) {
        newItem.Background = Brushes.CadetBlue;
        newItem.Foreground = Brushes.Yellow;
    } //if newItem
}; //lblDetailTree.SelectedItemChanged

Tested: CadetBlue does not look so nice, try Navy :-).

You cannot use strings as nodes (there is nothing to paint in this object :-)). Because of a very flexible content model of WPF, writing this effect for polymorphous objects or universal code insensitive to the node type is a bit difficult. For example, using direct base class of TextBlock is not possible, because this is FrameworkElement, which does not have Background or Foreground property, so, it you use a combination of controls and other framework elements, you will need to make a separate cast/check for every FrameworkElement-based class which is not Control and one for all Control-based classes.

—SA