Most efficient way to change CSS between pages
I think the usual method of adding per-page CSS is in your template.php file using HOOK_preprocess_page(). A couple of examples:

alphorn_preprocess_page(&amp;vars) {
  // Add CSS based on path (beware if you change your path structure!)
  // But as a bonus, this will add the CSS for any path under "/colour/red"
  if (strpos($_SERVER['REQUEST_URI'], 'colour/red') !== false) {
    drupal_add_css(drupal_get_path('theme','alphorn').'/css/red.css');
  }

  // Add CSS based on NID, with an out because this hook may be called on non-node pages
  // or pages that otherwise don't return anything useful for menu_get_object().
  @$node = menu_get_object();
  $red_nodes = array(1, 2, 3, 5, 8, 13);
  if(isset($node) &amp;&amp; in_array($node-&gt;nid, $red_nodes)) {
    drupal_add_css(drupal_get_path('theme','alphorn').'/css/red.css');
    // If your stylesheet is really simple you could always inline it.
    drupal_add_css('body { background-color: #FF0000; }', 'inline');
  }

  // Then you need to plug your added CSS into your variables or nothing will happen.
  $vars['styles'] = drupal_get_css();
}