Adding an drop down arrow to my site

I was wondering if there was a way to add a down arrow icon to the drop down menu? I've tried multiple css codes but nothing has worked. Any help would be appreciated!

My website is kashburahospitality.com

I don't see dropdown menu. Can you add it?

To add an arrow, add this code to Website Tools (under Not Linked) > Custom CSS

a.header-nav-folder-title:after {
    content: "\e009";
    font-family: 'squarespace-ui-font';
    position: relative;
    top: 2px;
}

@tuanphan how do I add a dropdown menu? 

Choose Folder