How to change sub menu icon from + to hamburger

I'm trying to change the '+' icon on the sub navigation menu as seen only on mobile devices, to the more familiar looking hamburger icon but can't seem to replace it easily.  Reference my page here: https://labattoir.ca/fall-winter-plated-menus on a mobile device to see what I'm talking about. 

I don't want to do this site wide just on one bunch of pages that live in a folder.  Anyone know how to do this?  Oh and I'm on the Hayden template.  Thanks!!

Add to Design > Custom CSS

/* Plus to burger */
.folder-nav-toggle:after, .category-nav-toggle:after {
    content: "\e030";
    font-family: 'squarespace-ui-font';
    font-size: 50px;
}