Github recently changed their default text font. This restores it to the previous settings -- Helvetica is prettier than Ubuntu Sans on Linux.

Github Helvetica.user.js

// ==UserScript==
// @name          Github Helvetica
// @namespace     http://userstyles.org
// @description	  Github recently changed their default text font. This restores it to the previous settings -- Helvetica is prettier than Ubuntu Sans on Linux.
// @author        Ryan Hitchman
// @homepage      https://userstyles.org/styles/130086
// @match         https://github.com/*
// @match         https://*.github.com/*
// @run-at        document-start
// @version       0.20160711225447
// ==/UserScript==
(function() {var css = [
	"body,.markdown-body,.intgrs-page .main-content,.blog-content,.migration-wrapper,.migrate-owners-wrapper,.setup-wrapper,.showcase-page-title,.tooltipped::after {",
	"    font-family: Helvetica, arial, nimbussansl, liberationsans, freesans, clean, sans-serif, \"Apple Color Emoji\", \"Segoe UI Emoji\", \"Segoe UI Symbol\";",
	"}"
].join("\n");
if (typeof addStyle != "undefined") {
	addStyle(css);
} else {
	var node = document.createElement("style");
	node.type = "text/css";
	node.appendChild(document.createTextNode(css));
	var heads = document.getElementsByTagName("head");
	if (heads.length > 0) {
		heads[0].appendChild(node); 
	} else {
		// no head yet, stick it whereever
		document.documentElement.appendChild(node);
	}
}
})();