Incase someone needs it:
/* Reduce opacity of all top-level menu items and dropdown menu items */
.header-nav-list:hover .header-nav-item,
.header-nav-list:hover .header-nav-folder-content .header-nav-folder-item {
opacity: 0.3;
}
/* Keep full opacity for the top-level menu item being hovered over */
.header-nav-list:hover > .header-nav-item:hover {
opacity: 1.0;
}
/* Keep full opacity for the dropdown menu items being hovered over */
.header-nav-folder-content:hover .header-nav-folder-item:hover {
opacity: 1.0;
}
/* Reduce opacity of the top-level menu item when hovering over its dropdown items */
.header-nav-folder-content:hover ~ .header-nav-item:hover {
opacity: 0.3;
}
