<?php
/**
 * Create your custom stylesheets based on the taxonomy tid in
 *  $theme_dir/css/taxonomy/$tid.css
 */
function THEME_preprocess_node(&$vars, $hook) {
  if ($terms = taxonomy_node_get_terms($vars->['node'])) {
    foreach ($terms as $term) {
      $stylesheet = '/css/taxonomy/'. $term->tid .'.css';
      if (file_exists(dirname(__FILE__) . $stylesheet)) {
        drupal_add_css(path_to_theme() . $stylesheet);
      }
    }
  }
}