twitter.font.user.js

// ==UserScript==
// @name        Twitter: Set custom font
// @version     2
// @grant       none
// @include     https://twitter.com/*
// @include     https://*.twitter.com/*
// ==/UserScript==

/* Change the font name in the apostrophes in the line below to your liking. */
var font = 'Courier New';

(function () {
  var head = document.getElementsByTagName('head')[0];
  var style = document.createElement('style');
  style.type = 'text/css';
  style.innerHTML = '* { font-family: \'' + font + '\' !important; }';
  head.appendChild(style);
})();