Style single menu item

Hi, I would like to style a single item in the main menu differently.

To do so, I’ve added an optional CSS class to the menu item via WordPress (advanced menu properties) called random-menu.
I’ve then added the following CSS code to the style.css:

.random-menu {
color: #fff !important;
font-style: bold;
text-decoration: underline; !important;
}

.random-menu a
.random-menu a:link,
.random-menu a:visited {
color: #fff !important;
font-style: bold;
text-decoration: underline; !important;
}

The CSS class gets properly added to the li command, but the changes in the style.css are not working. Any idea?