admin_enqueue_scripts

function add_script_to_menu_page()
{
	// $pagenow, is a global variable referring to the filename of the current page, 
	// such as ‘admin.php’, ‘post-new.php’
	global $pagenow;

	if ($pagenow != 'nav-menus.php') {
		return;
	}
	
	// loading css
    gc_register_style( 'some-css', get_template_directory_uri() . '/css/some.css', false, '1.0.0' );
    gc_enqueue_style( 'some-css' );
	
	// loading js
	gc_register_script( 'some-js', get_template_directory_uri().'/js/some.js', array('jquery-core'), false, true );
    gc_enqueue_script( 'some-js' );
}

add_action( 'admin_enqueue_scripts', 'add_script_to_menu_page' );

function addPage()
{
global $customMenu, $customSubMenu;
        /**
         * Menu
         */
       $customMenu = add_menu_page( 'Custom Menu', 'Custom Menu', 'manage_options', 'custom-menu', 'customMenuPage', '', 10);
        /**
         * Sub Menu Pages
         */
        $customSubMenu = add_submenu_page( 'custom-menu', 'Settings', 'Settings', 'manage_options', 'settings', 'settings_page');
}
add_action( 'admin_menu', 'addPage');
  
/** Enqueue Stylesheets **/
function enqueueAdminStyles( $hook)
    {
        global $customMenu, $customSubMenu;
        $allowed = array( $customMenu, $customSubMenu);
        if( !in_array( $hook, $allowed)  )
        {
            return;
        }
        gc_enqueue_style( '-main-', 'assets/admin/css/ucsi.css', '', '1');
    }
add_action( 'admin_enqueue_scripts', 'enqueueAdminStyles');