Rhye theme: filter registered cpt/taxonomy arguments

<?php

// Custom post types
add_filter( 'register_post_type_args', 'custom_filter_cpt_args', 10, 2 );
function custom_filter_cpt_args( $args, $post_type ) {
  if ( $post_type === 'arts_portfolio_item' ) {
    // Customizations for portfolio post type
  }

  return $args;
}

// Taxonomies
add_filter( 'register_taxonomy_args', 'custom_filter_taxonomy_args', 10, 2 );
function custom_filter_taxonomy_args( $args, $taxonomy ) {
  if ( $taxonomy === 'arts_portfolio_category' ) {
    // Customizations for categories taxonomy

    $args['hierarchical'] = true;
  }

  return $args;
}