A family of Microsoft word processing software products for creating web, email, and print documents.
The styles used in the index itself are Index 1 (for top-entries), Index 2 (for subentries), and so forth. It is those styles that you need to modify.
But what I had forgotten was that the index styles (by default) are set to update automatically, so just modifying one instance of any given style would change the formatting for all instances and for the style itself.