Just testing something :)

themes/list.php

 <?php
  $handle=opendir('themes');
  while ($file = readdir($handle)) {
-   if(!ereg("[.]",$file)) $themelist .= "$file ";
+   if(!ereg("^\.",$file) && file_exists("themes/$file/theme.class")) $themelist .= "$file ";
  }
  closedir($handle);
 ?>
\ No newline at end of file