The 4 preset styles for the vertical menus are basic, arrow, side and line. Each of them has different styles for the CLEAR, DARK, LIGHT or COLOR backgrounds:
To use the vertical menu styles, simply copy the name of the style inside the Menu Class Suffix field inside the Advanced options of the menu module:

For the CLEAR, DARK, LIGHT and COLOR vertical menu styles, add also menu-clear, menu-dark, menu-light or menu-color:
