Snippets: Active Menu Item When Viewing Portfolio Posts

Make sure to change the 300 to the ID of your menu item you are targeting. You can view this by inspecting your source code. If you prefer you can use PHP instead - https://wpexplorer-themes.com/total/snippets/current-menu-post-type-parent-pages/ - to add the current-menu-item class to the menu items instead.

body.single-portfolio #site-navigation .dropdown-menu > li#menu-item-300 > a {
 /* Your Custom CSS Here */
}
All CSS snippets should be added via a child theme's style.css file or using the built-in Custom CSS panel.