1: Find the minima bundle

bundle show minima

2: Inside the bundle (given by step 1), add the code block below to the end of _layouts/default.html (i.e. after the outmost <html> ... </html>)

<script type="text/x-mathjax-config">
  MathJax.Hub.Config({
    extensions: ["tex2jax.js"],
    jax: ["input/TeX", "output/HTML-CSS"],
    tex2jax: {
      inlineMath: [ ['$','$'], ["\\(","\\)"] ],
      displayMath: [ ['$$','$$'], ["\\[","\\]"] ],
      processEscapes: true
    },
    "HTML-CSS": { fonts: ["TeX"] }
  });
</script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-AMS-MML_HTMLorMML" type="text/javascript">
</script>

note:
- this step enables MathJax (with both inline and display mode)
  - $$...$$ is only rendered as display math if the lines above and below it are blank
- the above code uses the copy of MathJax from a Content Delivery Network cdnjs, which needs network access. alternative: download and install a local copy of MathJax on server/hard disk

3: Create a local copy of _layouts and save the change

cp -r {path to _layouts in the bundle} {path to repo}

How to enable MathJax in Jekyll minima theme

Reference: