Tampermonkey script for customizing the code UI.

customStyle.js

// ==UserScript==
// @name        Code customizer
// @description Custom css setter : 1. Set monospace font on websites 2. Set tab width
// @namespace   local.greasemonkey.codecustomizer
// @include     *
// @version     1
// @grant       none
// @author      [email protected]
// ==/UserScript==

const monoFont = "Jetbrains Mono, Fira Code, monospace" // Set your monospace font here.
const tabWidth = 2 // Set your tab width

// Function helper to inject css
function resetMonoFont() {
	Array.from(document.querySelectorAll("code, pre")).forEach(el => {
    let style = window.getComputedStyle(el)
    el.style.removeProperty('font-family')
    el.style.setProperty('font-family', monoFont, 'important')
  })
}

function addGlobalStyle(css) {
	let head, style
	head = document.getElementsByTagName('head')[0]
	if (!head) { return }
	style = document.createElement('style')
	style.type = 'text/css'
	style.innerHTML = css
	head.appendChild(style)
}

// Apply the font-family definition to code styles.
let selectors = [
	'code', // General
	'pre', // General
	'.monospace', // General
	'.blob-viewer', // Gitlab
	'.blob-code', // Github
	'.blob-num', // Github
	'.code', // General
	// 'td.code div.container code', // Geeksforgeeks,
]

//
let cssRules = `* { tab-size: ${tabWidth} !important; }`

for (let i = 0; i < selectors.length; i++) {
	cssRules = `${cssRules} ${selectors[i]}, ${selectors[i]} > * { font-family: ${monoFont} !important; }`
}

resetMonoFont()
addGlobalStyle(cssRules)