Customize TOC button

Here is a link to a blog post that could help:

My Learning's: Creating Custom Playbar skin in Captivate 9 (Standalone and HTML5)

However I totally agree with Justenuf... because I rarely use a default playbar. It is only appropriate for pure linear courses and I almost always have branching. Creating custom navigation and control buttons is not that hard, with shape buttons that can be used on master slides or timed for the rest of the project. Have a look at this blog post:

1 action = 5 Toggle Buttons - Captivate blog

If you want to dive into your JSON files after publication and find the playbar controls, you can reset/rename whatever you want - but it is re-coding inside this file and will require you to carefully find and replace all controls and calls for the playbar that you modify. I'd suggest simply creating a custom "playbar" at the bottom of your screen using smart shapes and advanced actions - unless you're really skilled in scripting, diving inside those files can lead to some interesting unintended consequences.

If you're dead set on customizing, contact Jim Leichtler or Captiv8tr on the forums and see if they can steer you through the shoals - good luck!