it is a little complicated cause you are using the minima theme which is a gem based theme - meaning the theme files are more or less hidden from you.
This post has some details on how to do it:
you will need to make 2 new directories in your root:
_sass - make a new file here named custom.scss and paste the code from the link above that had the css for the code highlighting
assets/css - make a file here called styles.scss with the following in it:
---
# Only the main Sass file needs front matter (the dashes are enough)
---
@import "minima-{{ site.minima.skin | default: 'classic' }}";
@import "custom";
seems like someone should add that to the minima theme.