CSS from Default Template and Admin Template gets combined when popup activated

Same thing happens when using the Sections module and the popup and the referrer page aren't in the same section. I've finally solved this with a custom hook_init() implementation. See code below. A corresponding implementation for the case of a separate admin theme should be a lot simpler. Hope this helps someone.

<?php
mymodule_init() {
  // Find out if this request is to be displayed in a popup.
  if ($_SERVER['HTTP_X_DRUPAL_RENDER_MODE'] == 'json/popups') {

    // We need the theme of this page to be the same as the referrer's theme,
    // otherwise conflicting CSS declaration will bring chaos and disorder.

    // Find the referrer's path.
    $q = str_replace(url(NULL, array('absolute' => TRUE)), '', $_SERVER['HTTP_REFERER']);
    $q_parts = explode('?', $q);
    $q = array_shift($q_parts);
    if (empty($q)) {
      $q = variable_get('site_frontpage', 'node');
    }

    // Check if this path is in a section ( taken from _sections_in_section() ).
    $section = NULL;
    global $user;
    $rids = array_keys($user->roles);
    $res = db_query(db_rewrite_sql('SELECT DISTINCT s.* FROM {sections_data} s LEFT JOIN {sections_roles} r ON s.sid = r.sid WHERE s.status = 1 AND (r.rid IN ('. db_placeholders($rids) .') OR r.rid IS NULL) ORDER BY s.weight', 's', 'sid'), $rids);
    while ($row = db_fetch_object($res)) {
      if ($row->visibility < 2) {
        $path = drupal_get_path_alias($_GET['q']);
        // Compare with the internal and path alias (if any).
        $page_match = drupal_match_path($path, $row->path);
        if ($path != $q) {
          $page_match = $page_match || drupal_match_path($q, $row->path);
        }
        // When $row->visibility has a value of 0, the block is displayed on
        // all pages except those listed in $row->path. When set to 1, it
        // is displayed only on those pages listed in $row->path. Prevent
        // the admin theme switching on block admin pages.
        if ($page_match = !($row->visibility xor $page_match) && !(drupal_match_path($path, "admin/build/block\nadmin/build/block/list/*"))) {
          $section = $row;
        }
      }
      else {
        if (drupal_eval($row->path)) {
          $section = $row;
        }
      }
    }
    
    global $custom_theme;
    if ($section) {
      // Set the theme to the referrer's section's theme.
      $custom_theme = $section->theme;
    }
    else {
      // Use the default theme if there is no section. (This popup may also
      // belong to a section.)
      $custom_theme = variable_get('theme_default', null);
    }
  }
}
?>