The Hereditor theme for Grav is a detached fork of the Mediator theme developed by Grav Team and contributors, which was a port of the Mediator theme for Jekyll designed by Dirk Fabisch, which in turn was inspired by the Readium theme for Ghost elaborated by Sven Read.
You can download the Hereditor theme individually on its own repository but it might be better to download the full Hereditor skeleton.
You can find out more info about customizing a Grav theme (as well as basic usage documentation on the official Grav Documentation) and check the full source code on GitHub.
The Hereditor theme is free and open source software, distributed under the MIT License version 2 or later. So feel free to to modify this theme to suit your needs.