Can I override a theme function from a contributed module
The page you are referring is for Drupal 5.

In Drupal 6, you alter a theme function defined in a module by defining the theme function, as in the following code. (Replace mytheme with the short name of your theme.)

// Drupal 6
function mytheme_guestbook_entry($uid, $entry, $comment_entry = NULL, $zebra, $confirm_delete = FALSE) {
  // Your own code.
}




// Drupal 7
function mytheme_guestbook_entry($variables) {
  // … 
}


In Drupal 7, if you just want to alter the variables passed to the theme function, you need to implement the preprocess function for that theme function.

function mymodule_preprocess_guestbook_entry(&amp;$variables) {
  // Alter the values passed to the function, such as $variables['uid'], $variables['entry'],
  // $variables['comment_entry'], $variables['zebra'], or $variables['confirm_delete'].
}


The code I shown works whatever a theme function is implemented in Drupal, or a third-party module.