To skin the aspdotnetstorefront vertical menu, you need update the css file in skins/skin_1/style.css.
Change the background-color for:
#vertmenustyle1
#vertmenustyle1 ul
#vertmenustyle1 li a
#vertmenustyle1 li a:hover
To skin the vertical menu, update:
.TopMenuItemHover
.MenuItemHover
.MenuItemHover td