Change menu classes

gistfile1.php

<?php 

function register_my_page() {
    global $my_page;
    $my_page = add_menu_page( 'My Page', 'My Page', 'edit_pages', 'my_page.php', 'my_page_hook', 'dashicons-groups', 4 );
}
add_action('admin_menu', 'register_my_page');

function highlight_different_menu($parent_file){

    global $current_screen;
    global $my_page;

    if ( $current_screen['base'] == $my_page ) {    
	     $parent_file = 'my_other_page.php';
    }

    return $parent_file;
}

add_filter('parent_file', 'highlight_different_menu');

Ok, so I've changed the original gist. When I register a menu item I store the hook that's returned in a global. I then compared that to the base of the current screen to set a new class.

Still doesn't solve the fact that $parent_file expects admin.php not admin.php?page=wpmark_content. In your example above you are setting the parent file to an already existing WordPress admin top level menu.

You should be able to just pass wpmark_content then (i.e. your menu slug), I was just giving an example in mine.

I always tend to add .php as per the codex example too. So wpmark_content.php

Unfortunately that does not work :-(