Hi,
The code you have posted should update the stroke style of all currently selected shapes. If you have only one shape selected, only it will be updated. Do you want to achieve something else? If yes, please elaborate.
One suggestion regarding your code is to use oNode.Style.StrokeStyle instead of ComposeStrokeStyle() in the Else clause of your loop's If statement as this is the correct way to get the local stroke style of the shape for modification.