This is SassDoc's default theme which means this is the theme that will be used when running SassDoc without a custom theme.
This theme uses Nunjucks as a template engine.
Because this is likely to be the most used theme of all, it is not as simple as a theme can get. For instance, there is quite a bit of logic in both index.js and the templates. Fortunately, you don't have to deal with that at all.
There are some possibilites to customise the theme's view. Essentially what's being displayed, what's not, and what are the project informations to be displayed in the header and footer.
To learn how to customise the theme's view, please read the documentation on SassDoc's site. Fear not! It's all about creating a configuration file. No big deal.