How to include your glue module's CSS last

custom_glue.module

/**
 * Of course, custom_glue is the name of the module. Substitute your own module's name.
 */

/**
 * Preprocess the wrapping HTML.
 *
 * @param array &$variables
 *   Template variables.
 */
function custom_glue_preprocess_html(&$variables) {
  $override = drupal_get_path('module', 'custom_glue') . "/custom_glue.css";

  $options = array(
    'group' => CSS_THEME + 1,
    'weight' => 999,
  );

  drupal_add_css($override, $options);
}