Hi Ben,

You can do this like this:
1. In Appearance -> Menus select your menu and then click "Screen options" and add a checkmark next to "CSS Classes":

2. Now edit the menu item and add a custom HTML class to it (ie "custom-menu-item") and save:


3. Now Go to Theme options -> Custom CSS 

You can add the following CSS (adjust it to your liking)

.nav-menu .custom-menu-item {
    background: pink; /* change background color */
}

.nav-menu .custom-menu-item > a {
    color: red!important; /* change link color */
}




Save theme options and you should see the changes on the frontend.


Kind regards,

Andrej