Change specific Navigation Folder link into a button

Site URL: https://panda-guava-l26n.squarespace.com/

I would like to make the last nav item, which is a folder, into a button. This is the code I've played with, but it doesn't work:

Add to Home > Design > Custom CSS

nav#mobileNavigation>div:last-child, nav#mainNavigation>div:last-child {
    background: white;
	padding: 5px;
}
nav#mobileNavigation>div:last-child>.folder-toggle, nav#mainNavigation>div:last-child>.folder-toggle {
    color: blue;
}

Thank so much. It worked great.

Diane