@dsm: Hello Ulrich,
Go to => Appearance => Customize => Additional CSS and add the following CSS Code.
/* To change the background color of menu item HELFEN and its sub menu item */
#menu-item-374, #secondary-menu ul.menu #menu-item-374 ul a {
background-color: red;
}
/* To change the hover background color of menu item HELFEN and its sub menu item */
#secondary-menu ul.menu #menu-item-374:hover > a, #secondary-menu ul.menu #menu-item-374 ul li:hover > a {
background: #54fb01;
}
You can change the color according to your choice.
Let me know if this works out!
Kind Regards,
Skandha