If you have (like me) problems to get code-highlighting to work, here is my setup that works good enough for me. I actually use the first 'prism'-way but also added a 'highlight.js'-way.

Go to the backend > code injection:

Prism:

header:

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.19.0/themes/prism-tomorrow.min.css" integrity="sha256-xevuwyBEb2ZYh4nDhj0g3Z/rDBnM569hg9Vq6gEw/Sg=" crossorigin="anonymous" />
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.19.0/plugins/line-numbers/prism-line-numbers.min.css" integrity="sha256-Afz2ZJtXw+OuaPX10lZHY7fN1+FuTE/KdCs+j7WZTGc=" crossorigin="anonymous" />

footer:

<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.19.0/prism.min.js" integrity="sha256-YZQM6/hLBZYkb01VYf17isoQM4qpaOP+aX96hhYrWhg=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.19.0/plugins/line-numbers/prism-line-numbers.min.js" integrity="sha256-hep5s8952MqR7Y79JYfCXZD6vQjVHs7sOu/ZGrs1OEQ=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.19.0/plugins/autoloader/prism-autoloader.min.js" integrity="sha256-WIuEtgHNTdrDT2obGtHYz/emxxAj04sJBdMhRjDXd8I=" crossorigin="anonymous"></script>
<script>document.body.className += ' ' + 'line-numbers';</script>

Highlight.js:

header:

<link rel="stylesheet"
	href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/10.0.2/styles/default.min.css" 
	integrity="sha256-48w2xk74a+0hWSZT2qyC/X5MNkwyyDRDNqoT99v1LJA=" 
	crossorigin="anonymous" />

footer:

<script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/10.0.2/highlight.min.js" 
	integrity="sha256-xBWd+VDTBasaTja2bfaCX4aA2H18UxRsjRFkK3rgfkI=" 
	crossorigin="anonymous">
</script>

<script>hljs.initHighlightingOnLoad();</script>

Both:

if you want to change the background-color of the code-section:

<style>
  .post-full-content pre {
    background: #eee;
  }
</style>

if you want to change the font size(e.g. by half):

<style>
  .post-full-content pre {
    font-size: 0.5em;
  }
</style>

Refs: