style-hack.php

<?php
$style = '';
foreach($styles as $stylesheet) {
  if (strpos($stylesheet['data'], 'system/system.menus.css') > 0) {
    // Skip this one, at least for my theme.
    continue;
  }
  // The following are nicked from Drupal core.
  $contents = drupal_load_stylesheet($stylesheet['data'], TRUE);

  // Build the base URL of this CSS file: start with the full URL.
  $css_base_url = file_create_url($stylesheet['data']);
  // Move to the parent.
  $css_base_url = substr($css_base_url, 0, strrpos($css_base_url, '/'));
  // Simplify to a relative URL if the stylesheet URL starts with the
  // base URL of the website.
  if (substr($css_base_url, 0, strlen($GLOBALS['base_root'])) == $GLOBALS['base_root']) {
    $css_base_url = substr($css_base_url, strlen($GLOBALS['base_root']));
  }

  _drupal_build_css_path(NULL, $css_base_url . '/');
  // Anchor all paths in the CSS with its base URL, ignoring external and absolute paths.
  $style .= preg_replace_callback('/url\(\s*[\'"]?(?![a-z]+:|\/+)([^\'")]+)[\'"]?\s*\)/i', '_drupal_build_css_path', $contents);
}