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');

Can you show me how you've added the custom page? add_menu_page()? add_submenu_page()?

Yes sure.

function wpmark_add_top_level_admin_menus() {

    /* if the user is a super admin */
    if( is_super_admin() )
        return;

    $wpmark_top_level_admin_menus = apply_filters( 'wpmark_top_level_admin_menus',
        array(

            /* this is the contnet menu item */
            array(
                'page_title' => 'Add Content',
                'menu_name' => 'Content',
                'capability' => 'edit_pages',
                'menu_slug' => 'wpmark_content',
                'content_function' => 'wpmark_content',
                'icon_location' => 'div',
                'menu_position' => 2,
            )

            /* other arrays in here doing other top level pages */

        )

    );

    /* loop through each menu item to add */
    foreach( $wpmark_top_level_admin_menus as $wpmark_top_level_admin_menu ) {

        /* add the menu page */
        add_menu_page(
            $wpmark_top_level_admin_menu[ 'page_title' ],
            $wpmark_top_level_admin_menu[ 'menu_name' ],
            $wpmark_top_level_admin_menu[ 'capability' ],
            $wpmark_top_level_admin_menu[ 'menu_slug' ],
            $wpmark_top_level_admin_menu[ 'content_function' ],
            $wpmark_top_level_admin_menu[ 'icon_location' ],
            $wpmark_top_level_admin_menu[ 'menu_position' ]
        );

    } // end foreach loop

}

add_action( 'admin_menu', 'wpmark_add_top_level_admin_menus' );

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 :-(