Add the Customizer to the Toolbar

add_action( 'admin_bar_menu', function( $wp_adminbar ) {

    global $wp;
    $current_url = home_url( add_query_arg( array(), $wp->request ) );

    $wp_adminbar->add_node( array(
        'id'    => 'customize',
        'title' => 'Customize',
        'href'  => add_query_arg( 'url', $current_url, admin_url( 'customize.php' ) ),
    ) );
}, 999 );