Documentation for the add-ons (or at least most of them) comes packaged with and can be accessed via the NVDA main menu.
NVDA+N,H,D (NVDA Main, Help, Run Add-ons Documentation) after pressing which you will be presented with a list of your installed add-ons for which a documentation package was included. Since the GitHub page for that add-on mentions updating the documentation as part of this latest version, the Audio Themes add-on should be somewhere in that list. I can't know where as I can't know what add-ons any given person might be using. Navigate to it, hit Enter, and the documentation will open in the web browser you have set as default on your system for your reading pleasure.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Power is being told you're not loved and not being destroyed by it.