Raw Source
mikhail5555 / Change all fonts to Calibri 

// ==UserScript==
// @name         Change all fonts to Calibri
// @description  Change annoying fonts to a more readable one
// @version      0.1.1
// @author       Mikhail5555
// @copyright    2018, Mikhail5555 (https://openuserjs.org/users/mikhail5555)
// @license      MIT
// @match        *://*/*
// @exclude      *://*.google.com/*
// @exclude      *://openuserjs.org/*
// @grant        GM_addStyle
// @grant        GM_getResourceText
// @resource     customCSS https://fonts.googleapis.com/css?family=Calibri
// @run-at       document-start
// @updateURL https://openuserjs.org/meta/mikhail5555/Change_all_fonts_to_Calibri.meta.js
// ==/UserScript==

var newCSS = GM_getResourceText("customCSS");
GM_addStyle(newCSS);

let font = 'Calibri,Candara,Segoe,Segoe UI,Optima,Arial,sans-serif';
let selectors = 'html, body, p, h1, h2, h3, h4, h5, label, div, a:not([class*="icon"]), b, cite, time, li, la, td, link, address, article, footer, main, nav, section, code, input, pre, select';
GM_addStyle(selectors + '{font-family: ' + font + ' !important;}');