background color

best would be to have a custom class on that element f.e. bg-colored-sub-menu

#top .av-submenu-container.bg-colored-sub-menu {
  background-color: lightblue;
}

this will only have an effect on the submenu – if you like to have the same bg-color for the list items:

#top .av-submenu-container.bg-colored-sub-menu a {
  background-color: lightblue;
  color: #000;
  border-color: darkblue
}