The CSS sections have been kept in the CSS files and the Js section have been kept in the .min.js files or just .js files.When trying edit CSS for a new style , first see the corresponding class names in the index.html file. Then go to the CSS file and search for that class name. You will usually find it there. And if you edit that class styles, you can edit the particular section of that webpage.
Most importantly , don't be afraid to break apart the code and see which section does what by trial and error. Tinkering with code is the fastest way by which you can learn stuff. If you cannot find a particlar class in the CSS file but you are sure that you saw that class name in the index.html file, google the class name. Maybe you will find that it is a predefined default class within CSS.
For example i didn't know why "fa-fa" was used in the social media sidebar and so i googled it. It was something related to Font Awesome. Basically styles.
So don't be afraid to contribute or afraid that you might botch up the code. If you happen to botch up the code , then just delete the repository from your Desktop Github version and again fork and download and start afresh. Happy coding.