The underlying code of the Author Theme is written in a way that’s easy to understand and modify for new and experienced developers alike.
However, if you edit any of the core theme files, including style.css, when the theme is updated your modifications will be overwritten.
Fortunately, there is a better way, by installing a child theme alongside the Author theme, you can customize any file without your changes being overwritten.
You can download a basic child theme below, which will allow you to add custom css code (style.css) and custom php functions (functions.php).
Using a child theme you can also override any core theme file. For example, if you wanted to make changes to index.php. You can simply copy that file into your child theme folder, then make your edits.
Child theme files will always be given priority over core theme files, so using this method you can modify any part of the theme, while still being able to update to the latest version of the Author theme.
Child Theme Download: https://olympusthemes.com/child-themes/ot-author-child.zip