hello,
I could find a way to add a white line underneath each menu item, but the width only spanned the width of the text, rather than the width of the menu box.
To be honest, the Treeview menu is not that easy to style, if you wish to create a vertical menu, I would look at a CSS menu as it is much easier to set the stylings that you require with a CSS menu. (and they are search engine friendly)
CSS menus that are available are the HouseMenu (free) and the Snapsis menu (purchase)
I have created tutorials on how to create a vertical house menu here:
How to create a pure CSS skin (video 7)
In that tutorial I used house menu skin object v1.3
Thanks,