Greasemonkey script to make GitHub widescreen friendly. There's probably some quirks. Use at your own risk.

github_widescreen

// ==UserScript==
// @name           GitHub Widescreen
// @namespace      https://github.com
// @include        https://github.com/*
// @grant          none
// ==/UserScript==
(function() {
	var style = document.createElement("style");
	style.type = "text/css";
	style.innerHTML = "\
.container { \
	position: relative; \
	width: 100% !important; \
} \
.container > .repository-content { \
	margin: 0 auto; \
	width: 98% !important; \
} \
.header > .container, \
.pagehead, \
.org-header > .container, \
.org-main, \
.page-content, \
.container > .site-footer { \
	padding-left: 2%; \
	padding-right: 2%; \
} \
.org-main { \
	margin-right: 300px; \
} \
.org-sidebar { \
	position: absolute; \
	top: 0; \
	right: 2%; \
} \
.blob-code { \
	-moz-tab-size: 4 !important; \
	tab-size: 4 !important; \
} \
table.files td.content { \
	width: 25%; \
} \
";
	document.getElementsByTagName("head")[0].appendChild(style);
}());

Great, just what I needed! Thank you!
Tip: Look up Stylish for your browser. It's easier to do style-only mods with that.