Improved version of Mou GitHub2 CSS for Markdown. Using Menlo for code and fixed some bugs.

GitHub2 Edit.css

body {
	font-family: Helvetica, sans-serif;
	font-size: 14px;
	line-height: 1.6;
	padding-top: 10px;
	padding-bottom: 10px;
	background-color: white;
	padding: 30px;
}

body > *:first-child {
	margin-top: 0 !important;
}

body > *:last-child {
	margin-bottom: 0 !important;
}

a {
	color: #4183c4;
	/* can not remove underline */
}

em {
	font-family: "Helvetica Neue", sans-serif; /* for better contrast to normal text */
}

a.absent {
	color: #cc0000;
}

a.anchor {
	display: block;
	padding-left: 30px;
	margin-left: -30px;
	cursor: pointer;
	position: absolute;
	top: 0;
	left: 0;
	bottom: 0;
}

h1, h2, h3, h4, h5, h6 {
	margin: 20px 0 10px;
	padding: 0;
	font-weight: bold;
	-webkit-font-smoothing: antialiased;
	cursor: text;
	position: relative;
}

h1:hover a.anchor, h2:hover a.anchor, h3:hover a.anchor, h4:hover a.anchor, h5:hover a.anchor, h6:hover a.anchor {
	background: url(data:image/png;base64,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) no-repeat 10px center;
	text-decoration: none;
}

h1 tt, h1 code, h2 tt, h2 code, h3 tt, h3 code, h4 tt, h4 code, h5 tt, h5 code, h6 tt, h6 code {
	font-size: inherit;
}

h1 {
	font-size: 32px;
	color: black;
}

h2 {
	font-size: 24px;
	border-bottom: 1px solid #ddd;
	color: black;
}

h3 {
	font-size: 18px;
}

h4 {
	font-size: 16px;
}

h5 {
	font-size: 14px;
}

h6 {
	color: #777777;
	font-size: 14px;
}

p, blockquote, ul, ol, dl, li, table, pre {
	margin: 15px 0;
}

ul ul, ul ol, ol ol, ol ul {
	margin-top: 0;
	margin-bottom: 0;
}

hr {
	background: transparent url(data:image/png;base64,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) repeat-x 0 0;
	border: 0 none;
	color: #ddd;
	height: 4px;
	padding: 0;
}

body > h2:first-child {
	margin-top: 0;
	padding-top: 0;
}

body > h1:first-child {
	margin-top: 0;
	padding-top: 0;
}

body > h1:first-child + h2 {
	margin-top: 0;
	padding-top: 0;
}

body > h3:first-child, body > h4:first-child, body > h5:first-child, body > h6:first-child {
	margin-top: 0;
	padding-top: 0;
}

a:first-child h1, a:first-child h2, a:first-child h3, a:first-child h4, a:first-child h5, a:first-child h6 {
	margin-top: 0;
	padding-top: 0;
}

h1 p, h2 p, h3 p, h4 p, h5 p, h6 p {
	margin-top: 0;
}

li p.first {
	display: inline-block;
}

li {
	margin: 0;
}

ul, ol {
	padding-left: 30px;
}

ul :first-child, ol :first-child {
	margin-top: 0;
}

dl {
	padding: 0;
}

dl dt {
	font-size: 14px;
	font-weight: bold;
	font-style: italic;
	padding: 0;
	margin: 15px 0 5px;
}

dl dt:first-child {
	padding: 0;
}

dl dt > :first-child {
	margin-top: 0;
}

dl dt > :last-child {
	margin-bottom: 0;
}

dl dd {
	margin: 0 0 15px;
	padding: 0 15px;
}

dl dd > :first-child {
	margin-top: 0;
}

dl dd > :last-child {
	margin-bottom: 0;
}

blockquote {
	border-left: 4px solid #ddd;
	padding: 0 15px;
	color: #777;
}

blockquote > :first-child {
	margin-top: 0;
}

blockquote > :last-child {
	margin-bottom: 0;
}

table {
	padding: 0;
	border-collapse: collapse;
}

table tr {
	border-top: 1px solid #ddd;
	background-color: white;
	margin: 0;
	padding: 0;
}

table tr:nth-child(2n) {
	background-color: #f8f8f8;
}

table tr th {
	font-weight: bold;
	border: 1px solid #ddd;
	margin: 0;
	padding: 6px 13px;
}

table tr td {
	border: 1px solid #ddd;
	margin: 0;
	padding: 6px 13px;
}

table tr th :first-child, table tr td :first-child {
	margin-top: 0;
}

table tr th :last-child, table tr td :last-child {
	margin-bottom: 0;
}

img {
	max-width: 100%;
	height: auto;
}

span.frame {
	display: block;
	overflow: hidden;
}

span.frame > span {
	border: 1px solid #ddd;
	display: block;
	float: left;
	overflow: hidden;
	margin: 13px 0 0;
	padding: 7px;
	width: auto;
}

span.frame span img {
	display: block;
	float: left;
}

span.frame span span {
	clear: both;
	color: #333333;
	display: block;
	padding: 5px 0 0;
}

span.align-center {
	display: block;
	overflow: hidden;
	clear: both;
}

span.align-center > span {
	display: block;
	overflow: hidden;
	margin: 13px auto 0;
	text-align: center;
}

span.align-center span img {
	margin: 0 auto;
	text-align: center;
}

span.align-right {
	display: block;
	overflow: hidden;
	clear: both;
}

span.align-right > span {
	display: block;
	overflow: hidden;
	margin: 13px 0 0;
	text-align: right;
}

span.align-right span img {
	margin: 0;
	text-align: right;
}

span.float-left {
	display: block;
	margin-right: 13px;
	overflow: hidden;
	float: left;
}

span.float-left span {
	margin: 13px 0 0;
}

span.float-right {
	display: block;
	margin-left: 13px;
	overflow: hidden;
	float: right;
}

span.float-right > span {
	display: block;
	overflow: hidden;
	margin: 13px auto 0;
	text-align: right;
}

code, tt {
	font-family: Menlo, Consolas, Monaco, "Lucida Console", monospace;
	font-size: 12px;
	margin: 0 2px;
	padding: 1px 5px;
	white-space: nowrap;
	border: 1px solid #ddd;
	background-color: #f8f8f8;
	border-radius: 3px;
}

pre code {
	margin: 0;
	padding: 0;
	white-space: pre;
	border: none;
	background: transparent;
}

.highlight pre {
	background-color: #f8f8f8;
	border: 1px solid #ddd;
	font-size: 13px;
	line-height: 19px;
	overflow: auto;
	padding: 6px 10px;
	border-radius: 3px;
}

pre {
	background-color: #f8f8f8;
	border: 1px solid #ddd;
	font-size: 13px;
	line-height: 19px;
	overflow: auto;
	padding: 6px 10px;
	border-radius: 3px;
}

pre code, pre tt {
	background-color: transparent;
	border: none;
}

sup {
	font-size: 0.83em;
	vertical-align: super;
	line-height: 0;
}

* {
	-webkit-print-color-adjust: exact;
}

@media screen and (min-width: 914px) {
	body {
		width: 854px;
		margin: 0 auto;
	};
}

@media print {
	table, pre {
		page-break-inside: avoid;
	}

	pre {
		word-wrap: break-word;
	};
}

Download Mou markdown editor for Mac here.