Adding a border around navigation items (mobile nav items only if possible)

Site URL: https://www.eatatbirdhouse.com/

I have a client who has requested I put a burnt orange border - color (#ad472d) around the navigation items on the hamburger menu when you expand the items. They do not want the border on the computer view just the mobile + tablet navigation. I have tried inserting some custom CSS from other forums with no luck. Any and all feed back is much appreciated. I have an attached an image of the navigation where they would like to see red borders around each item. 

Add to Home > Design > Custom CSS

nav#mobileNavigation a {
    border-top: 1px solid orange;
}
nav#mobileNavigation>div:last-child a {
    border-bottom: 1px solid orange;
}