Inserting a Divider After Categories and Thier Sub-Forms

I have seperated my categories and their sub-forms as one entity. I wish to have a divider placed after these categories. The html code for the divider and the divider graphic itself have already been prepared.

I attempted to paste in my custom html following the code found in the forumhome_forumbit_level2_post but the result was the divider appearing after each subform. I assume there is a loop function that lists the sub-forms under the category headings in forumdisplay.php? If so, on which line in that file can I insert my code to ensure the divider is correctly positioned?