Dropdown Menu Icon On Squarespace

STEP 1: adding this to Settings > Advanced > Code Injection > Header:

<link href="//maxcdn.bootstrapcdn.com/font-awesome/4.5.0/css/font-awesome.min.css" rel="stylesheet">

STEP 2:And add this to Design > Custom CSS:

.header-nav-item.header-nav-item--folder a.header-nav-folder-title:after {
  font-family: FontAwesome;
  content: "\f107";
  padding-left: 4px;
}