Theming the "not-so-apparent" parts of Drupal content

An excerpt from the above-linked article/resource:
"You can create theme hooks for modules you did not write
When I wanted to theme the comments form the first thing I did was looking at comment.module. There I found a call like this:

drupal_get_form('comment_form', $edit, $title) 

Ok, I had a comment_form function that I could theme according to Drupal Form API, then I started looking for theme_comment_form but there was none.
How to override a theme hook that does not exist? Just as you do with your own modules, you just need to know the code involved and, this is important, you can include a hook_theme function in your template.php file. This is what I did for Woodpig:

function woodpig_theme() {
return array(
'comment_form' => array(
'arguments' => array('form' => array()),
),
);
} 

The function starts with the theme name and the theme hook takes the form argument because it's a that's what drupal_get_form requires. Then I can write my own woodpig_comment_form($form) function."