Change one menu-item to green background

Hi.

I want to change one of the menu items (kontakt os for tilbud) in the navigation to having a green background. So far I’ve tried stuff like inserting this line in the hvid.css file:

.menu-item menu-item-type-post_type menu-item-object-page menu-item-23 {background-color: #7ec642;}

This doesn’t work though. Anyone that has ideas on how I can do this?

URL: https://www.diction.dk