function _phptemplate_variables($hook, $vars) {
  switch ($hook) {
    case 'page':
      $vars['css'] = _sympal_theme_unset_css($vars['css']);
      $vars['styles'] = drupal_get_css($vars['css']);
    break;
  }
 
  return $vars;
}

/**
 * Unset all module and core styles
 **/
function _sympal_theme_unset_css($css) {
  if (is_array($css['all']['module'])) {
    $css['all']['module'] = array();
  }
  return $css;
}

Removes the core, and node styles from the $style variable. 

You most morbably already defined _phptemplate_variables($hook, $vars), in that case just add the two lines there.
_sympal_theme.. is the name of the theme where we use this.

Drupal - Unset (not include) all core stylesheets - Theme variable overrides.

css, drupal, theme

PHP