Add 'Customize' link under Appearance

gistfile1.php

function ww_add_customize_link() {
    // add the Customize link to the admin menu
    add_theme_page( 'Customize', 'Customize', 'edit_theme_options', 'customize.php' );
}

add_action ( 'admin_menu', 'ww_add_customize_link' );