Welcome @hosios
You might find it easier to download the skeleton and install in to a fresh directory
https://github.com/getgrav/grav-skeleton-rtfm-site or paul hibbits
https://github.com/hibbitts-design/grav-skeleton-learn2-with-git-sync
Also some more information can be found here on a similar question where Paul has replied in depth.
If you want to use the theme and then use ‘inheritance’ of the template, it allows you to adapt it further without affecting if the themes get updated.
which will have pages showing you the data layout etc.
The basic information can be find in the template under the \user\templates\partials\sidebar.html.twig
But there is a a Table of Contents plugin (check the downloads → plugin and search for TOC which might be easier than recreating / dissecting the code.
After a quick looking at the theme. the code which produces the loop to go through the pages can be found here
It appears in the sidebar in the learn twig, under partials
templates/partials/sidebar.html.twig
the css styling can be found in the scss or css-compiled under
learn/blob/develop/themes/learn/css-compiled/template.css around line 2122)