I don't have Styler open in front of me, and honestly, I can't remember whether or not I've coded an index in Styler before or not. (I definitely have indexes, but they might all be FOSI based.) So this will be a strategy tip not a step by step tip.
What you will want to look for are Styler formatting elements or SFEs. You may be familiar with User formatting elements or UFEs. Very similar. Depeding on what version of Styler you are using, how you find and format these will be different. There is a menu selection, I believe, that will toggle them on/off in your element map. Anyhow, I have definitely played with the Table of Contents SFEs to control/change this sort of behavior.
Your best bet, if that tip doesn't get you all the way there and you haven't moved on already, is to join the Adepters mailing list where lots and lots of Arbortext users, admins, and developers hang out 24/7/366 (it is a leap year, you know!). Information on joining that list (as well as pointers to lots of other Arbortext resources) can be found here: