I have never seen editing the format of a table of contents. However, since you are in version 188.8.131.52, it may worth reading the http://docs.oracle.com/cd/E40248_01/epm.1112/fr_webuser.pdf
I paste the wrong document.... http://docs.oracle.com/cd/E40248_01/epm.1112/fr_webadmin.pdf In the web admin, page 27, it says: psbtreeview.css — Use this file to define the textual appearance in the Table of Contents such as color.
Can you please check and let us know?
I have raised an SR for it, they mentioned few files changes, I'll post the solution after I verify if that is working, and that solution involved dealing with template.rtf file.
But i am surprised that in webadmin guide it said about a css file which defines TOC (as mentioned by Thanos). I'll have a talk with oracle on it as well.