Also see Beginners guide to overriding themable output.
terms (lists of child nodes of taxonomy terms)
Stock Drupal styling
i.e. from a URL like taxonomy/term/4
You can change the term description and css by overriding the function
theme_taxonomy_term_page($tids, $result)
You want to change the contents of a row in the display? You are way out of luck.
The rows are rendered using an unthemed function taxonomy_render_nodes($result).
Several options then,
Use the Views module. Put anything in the rows then.
Use a taxonomy field extension module e.g. ?.
(A little like the above), there are Content Construction Kit type modules for terms. I don't know if or how they might work with taxonomy Treemenu.
Hang on for Drupal 7, scheduled to have field additions to term lists?
Treemenu unformatted styling
Three overridable theme functions
theme_taxonomy_treemenu_term_unformatted_page($term, $result)
$term presently has tid, name, and description fields.
theme_taxonomy_treemenu_term_row_data($data)
$data is the result of a taxonomy treemenu query on nodes. Presently, has sticky, title, and created fields.
theme_taxonomy_treemenu_term_row($row)
All in taxonomy_treemenu/includes/taxonomy_treemenu.pages.inc