Adding PrismJS to a theme without touching the theme itself

This blog uses PrismJS for syntax coding but everytime I switch themes, it can be difficult to modify. For example, I use Ghost for my CMS and if I use any of the built-in themes, it's not very easy to modify them. Therefore, here is my approach to adding syntax coding without having to modify the theme at all. I simply paste this code within the Site Header or Site Footer.

Step 1 - Use cdnjs to get the latest version of PrismJS

<!-- PRISM JS for Syntax Coding -->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.29.0/themes/prism-twilight.min.css" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.29.0/prism.min.js"></script>

Step 2 - Add any PrismJS plugins

<!-- PRISM JS Line Numbers Plugin -->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.29.0/plugins/line-numbers/prism-line-numbers.min.css" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.29.0/plugins/line-numbers/prism-line-numbers.min.js"></script>

Step 3 - Customize

A lot of my code blocks have use back ticks but do not include the prefix language-. Therefore, upon each page load, I need to dynamically check to see if there are any classes within the <code> block and if not then append a language prefix.

<script>
// Use the JSON Coloring to later add it to language-bash
Prism.languages.json = {
	'property': {
		pattern: /(^|[^\\])"(?:\\.|[^\\"\r\n])*"(?=\s*:)/,
		lookbehind: true,
		greedy: true
	},
	'string': {
		pattern: /(^|[^\\])"(?:\\.|[^\\"\r\n])*"(?!\s*:)/,
		lookbehind: true,
		greedy: true
	},
	'comment': {
		pattern: /\/\/.*|\/\*[\s\S]*?(?:\*\/|$)/,
		greedy: true
	},
	'number': /-?\b\d+(?:\.\d+)?(?:e[+-]?\d+)?\b/i,
	'punctuation': /[{}[\],]/,
	'operator': /:/,
	'boolean': /\b(?:false|true)\b/,
	'null': {
		pattern: /\bnull\b/,
		alias: 'keyword'
	}
};    
Prism.languages.bash = Prism.languages.json;

// Convert every <code> into <code class=language-powershell>
var codeArray = document.getElementsByTagName('code');
for(var i = 0; i < codeArray.length; i++){
    // If missing language, add one
    if(codeArray[i].classList == ''){
	    codeArray[i].classList.add("language-bash");
    }
    // Add Line Numbers to Code
	codeArray[i].classList.add("line-numbers");
    console.log(`${i} ${ codeArray[i].classList }`);
}
</script>