Custom Capability for especifc wp user

gistfile1.php

<?php
// Super User
function add_theme_caps() {
//to add capability to user
$user = new WP_User( $user_id = 1 ); //ID of user
$user->add_cap( 'custom_apability');
}
add_action( 'admin_init', 'add_theme_caps');
//