alisspers/current-menu-item-for-single-cpt-functions.php

## current-menu-item-for-single-cpt-functions.php
<?php
add_action(
	'init',
	function ()
	{
		// Vår meny
		register_nav_menu( 'main-menu', 'Huvudmeny' );

		// Custom Post Type för event
		$labels = array(
			'name'               => 'Events',
			'singular_name'      => 'Event',
			'all_items'          => 'All Events',
			'add_new_item'       => 'Add Event',
			'edit_item'          => 'Edit Event',
			'new_item'           => 'New Event',
			'view_item'          => 'View Event',
			'search_items'       => 'Search Events',
			'not_found'          => 'No events found',
			'not_found_in_trash' => 'No events found in trash',
		);

		register_post_type(
			'event',
			array(
				'labels'   => $labels,
				'supports' => array( 'title', 'editor', 'excerpt', 'thumbnail' ),
				'public'   => true,
			)
		);
	}
);


add_filter(
	'nav_menu_css_class',
	function ( $classes, $menu_item )
	{
		if ( is_singular( 'event' ) && in_array( 'events', $classes ) )
		{
			$classes[] = 'current-menu-item';
		}

		return $classes;
	},
	10,
	2
);

## functions.php
<?php
add_action(
	'init',
	function ()
	{
		// Vår meny
		register_nav_menu( 'main-menu', 'Huvudmeny' );

		// Custom Post Type för event
		$labels = array(
			'name'               => 'Events',
			'singular_name'      => 'Event',
			'all_items'          => 'All Events',
			'add_new_item'       => 'Add Event',
			'edit_item'          => 'Edit Event',
			'new_item'           => 'New Event',
			'view_item'          => 'View Event',
			'search_items'       => 'Search Events',
			'not_found'          => 'No events found',
			'not_found_in_trash' => 'No events found in trash',
		);

		register_post_type(
			'event',
			array(
				'labels'   => $labels,
				'supports' => array( 'title', 'editor', 'excerpt', 'thumbnail' ),
				'public'   => true,
			)
		);
	}
);

## header.php
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
	<title><?php bloginfo( 'title' ); ?></title>
	<?php
	wp_enqueue_style( 'wptemp', '/wp-content/themes/webbgaraget/style.css' );
	wp_head();
	?>
</head>
<body <?php body_class(); ?>>
	<header>
		<h1>Markera menyval i singelsidor för Custom Post Types</h1>
		<p>Meny:</p>
		<nav>
			<?php wp_nav_menu( array( 'theme_location'  => 'main-menu' ) ); ?>
		</nav>
	</header>
	<section role="main">


## style-2.css
/*
Theme Name: Markera menyval för CPT
Theme URI:
Description: Visar hur du på två olika sätt kan markera menyval vid visning av custom post types
Author: Webbgaraget
Author URI: http://www.webbgaraget.se/
Version: 1.0
*/

.current-menu-item a,
.single-event .menu-item.events {
    font-weight: bold;
}

## style.css
/*
Theme Name: Markera menyval för CPT
Theme URI:
Description: Visar hur du på två olika sätt kan markera menyval vid visning av custom post types
Author: Webbgaraget
Author URI: http://www.webbgaraget.se/
Version: 1.0
*/

.current-menu-item a {
    font-weight: bold;
}
	<?php
	add_action(
	'init',
	function ()
	{
	// Vår meny
	register_nav_menu( 'main-menu', 'Huvudmeny' );

	// Custom Post Type för event
	$labels = array(
	'name' => 'Events',
	'singular_name' => 'Event',
	'all_items' => 'All Events',
	'add_new_item' => 'Add Event',
	'edit_item' => 'Edit Event',
	'new_item' => 'New Event',
	'view_item' => 'View Event',
	'search_items' => 'Search Events',
	'not_found' => 'No events found',
	'not_found_in_trash' => 'No events found in trash',
	);

	register_post_type(
	'event',
	array(
	'labels' => $labels,
	'supports' => array( 'title', 'editor', 'excerpt', 'thumbnail' ),
	'public' => true,
	)
	);
	}
	);
	/*
	Theme Name: Markera menyval för CPT
	Theme URI:
	Description: Visar hur du på två olika sätt kan markera menyval vid visning av custom post types
	Author: Webbgaraget
	Author URI: http://www.webbgaraget.se/
	Version: 1.0
	*/

	.current-menu-item a,
	.single-event .menu-item.events {
	font-weight: bold;
	}