update your template.php then add preferred style to css

/* add item-odd item-even class to list */
function phptemplate_menu_item($link, $has_children, $menu = '', $in_active_trail = FALSE, $extra_class = NULL) {

static $count = 0;
$zebra = ($count % 2) ? 'even' : 'odd';
$count++;

  $class = ($menu ? 'expanded' : ($has_children ? 'collapsed' : 'leaf'));
  if (!empty($extra_class)) {
    $class .= ' '. $extra_class;
  }
  if ($in_active_trail) {
    $class .= ' active-trail';
  }
  $class .= ' item-'. $zebra;
  return '<li class="'. $class .'">'. $link . $menu ."</li>\n";
}