Under the appearance admin menu, I have customizer added by the theme, and theme options added by a plugin.  I'm using this code to hide both menus ( submenu's of Appearance )for ALL admins apart from a certain USERNAME. 

    function hide_menu() {
        global $current_user;
    	$current_user = wp_get_current_user();
    	$user_name = $current_user->user_login; 
    
    		//check condition for the user means show menu for this user
    		if(is_admin() &&  $user_name != 'USERNAME') {
    
         	 remove_submenu_page( 'themes.php', 'customize' );
    		 remove_submenu_page( 'themes.php', 'core-settings' );
       }
    }
    add_action('admin_head', 'hide-menu');

The code its self works fine, I can hide parent menu's.  But I can't seem to hide the two sub menu's that I want to hide. 

The two menu's point to URLs : 

    domainname.com/wp-admin/customize.php

    domainname.com/wp-admin/themes.php?page=core-settings

Submenu Debug :

    [6] => Array
                    (
                        [0] => Customise
                        [1] => customize
                        [2] => customize.php?return=%2Fwp-admin%2Findex.php
                        [3] => 
                        [4] => hide-if-no-customize
                    )
                   [21] => Array
                    (
                        [0] => Theme Settings
                        [1] => manage_options
                        [2] => core-settings
                        [3] => Theme Settings
                    )

I've looked at dubug mode but im not sure what im looking at, could someone please give a solution and an explanation as to why it works.