Custom submenu color

I’d like to change the color of a single submenu item–that is, there is one submenu page that refers to environmentalism, so the client wants the background of that single submenu button to be green.

I am using a custom child theme of TwentyEleven.

I found the place in the custom menu options to add a css style, but I can’t find where in the css to get this to apply. When I just create a single css style–

.earth-green {
  background-color: #006600;
}

–it covers over the parent menu button as well. I tried several combos with the above style and ul, a, etc, to no avail. What css syntax do I need to make this work?

Thanks!