(Applies to: all versions of FusionBB)
Customizing the rules for your forum is easily done by going to the following area:
My Fusion >> Admin Area >> Edit >> Inserts
Scroll down the page to the section second from the bottom: this is the Rules insert section and where you can modify the content. Please be aware that the forum rules text is inserted directly into the HTML page. If you want to use a special formatting to set off part of your text, you will need to use HTML tags to do so.
(Thanks to FusionBB members Alfonso for posing the question and Greg & Honda Guru for their helpful insight and answers)