GitHub-like CSS for pandoc standalone HTML files (perfect for HTML5 output). Based on Marked.app's GitHub CSS. Added normalize.css (v2.1.3) in the prior to GitHub css.

github-pandoc.css

/*! normalize.css v2.1.3 | MIT License | git.io/normalize */

/* ==========================================================================
   HTML5 display definitions
   ========================================================================== */

/**
 * Correct `block` display not defined in IE 8/9.
 */

article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
main,
nav,
section,
summary {
    display: block;
}

/**
 * Correct `inline-block` display not defined in IE 8/9.
 */

audio,
canvas,
video {
    display: inline-block;
}

/**
 * Prevent modern browsers from displaying `audio` without controls.
 * Remove excess height in iOS 5 devices.
 */

audio:not([controls]) {
    display: none;
    height: 0;
}

/**
 * Address `[hidden]` styling not present in IE 8/9.
 * Hide the `template` element in IE, Safari, and Firefox < 22.
 */

[hidden],
template {
    display: none;
}

/* ==========================================================================
   Base
   ========================================================================== */

/**
 * 1. Set default font family to sans-serif.
 * 2. Prevent iOS text size adjust after orientation change, without disabling
 *    user zoom.
 */

html {
    font-family: sans-serif; /* 1 */
    -ms-text-size-adjust: 100%; /* 2 */
    -webkit-text-size-adjust: 100%; /* 2 */
}

/**
 * Remove default margin.
 */

body {
    margin: 0;
}

/* ==========================================================================
   Links
   ========================================================================== */

/**
 * Remove the gray background color from active links in IE 10.
 */

a {
    background: transparent;
}

/**
 * Address `outline` inconsistency between Chrome and other browsers.
 */

a:focus {
    outline: thin dotted;
}

/**
 * Improve readability when focused and also mouse hovered in all browsers.
 */

a:active,
a:hover {
    outline: 0;
}

/* ==========================================================================
   Typography
   ========================================================================== */

/**
 * Address variable `h1` font-size and margin within `section` and `article`
 * contexts in Firefox 4+, Safari 5, and Chrome.
 */

h1 {
    font-size: 2em;
    margin: 0.67em 0;
}

/**
 * Address styling not present in IE 8/9, Safari 5, and Chrome.
 */

abbr[title] {
    border-bottom: 1px dotted;
}

/**
 * Address style set to `bolder` in Firefox 4+, Safari 5, and Chrome.
 */

b,
strong {
    font-weight: bold;
}

/**
 * Address styling not present in Safari 5 and Chrome.
 */

dfn {
    font-style: italic;
}

/**
 * Address differences between Firefox and other browsers.
 */

hr {
    -moz-box-sizing: content-box;
    box-sizing: content-box;
    height: 0;
}

/**
 * Address styling not present in IE 8/9.
 */

mark {
    background: #ff0;
    color: #000;
}

/**
 * Correct font family set oddly in Safari 5 and Chrome.
 */

code,
kbd,
pre,
samp {
    font-family: monospace, serif;
    font-size: 1em;
}

/**
 * Improve readability of pre-formatted text in all browsers.
 */

pre {
    white-space: pre-wrap;
}

/**
 * Set consistent quote types.
 */

q {
    quotes: "\201C" "\201D" "\2018" "\2019";
}

/**
 * Address inconsistent and variable font size in all browsers.
 */

small {
    font-size: 80%;
}

/**
 * Prevent `sub` and `sup` affecting `line-height` in all browsers.
 */

sub,
sup {
    font-size: 75%;
    line-height: 0;
    position: relative;
    vertical-align: baseline;
}

sup {
    top: -0.5em;
}

sub {
    bottom: -0.25em;
}

/* ==========================================================================
   Embedded content
   ========================================================================== */

/**
 * Remove border when inside `a` element in IE 8/9.
 */

img {
    border: 0;
}

/**
 * Correct overflow displayed oddly in IE 9.
 */

svg:not(:root) {
    overflow: hidden;
}

/* ==========================================================================
   Figures
   ========================================================================== */

/**
 * Address margin not present in IE 8/9 and Safari 5.
 */

figure {
    margin: 0;
}

/* ==========================================================================
   Forms
   ========================================================================== */

/**
 * Define consistent border, margin, and padding.
 */

fieldset {
    border: 1px solid #c0c0c0;
    margin: 0 2px;
    padding: 0.35em 0.625em 0.75em;
}

/**
 * 1. Correct `color` not being inherited in IE 8/9.
 * 2. Remove padding so people aren't caught out if they zero out fieldsets.
 */

legend {
    border: 0; /* 1 */
    padding: 0; /* 2 */
}

/**
 * 1. Correct font family not being inherited in all browsers.
 * 2. Correct font size not being inherited in all browsers.
 * 3. Address margins set differently in Firefox 4+, Safari 5, and Chrome.
 */

button,
input,
select,
textarea {
    font-family: inherit; /* 1 */
    font-size: 100%; /* 2 */
    margin: 0; /* 3 */
}

/**
 * Address Firefox 4+ setting `line-height` on `input` using `!important` in
 * the UA stylesheet.
 */

button,
input {
    line-height: normal;
}

/**
 * Address inconsistent `text-transform` inheritance for `button` and `select`.
 * All other form control elements do not inherit `text-transform` values.
 * Correct `button` style inheritance in Chrome, Safari 5+, and IE 8+.
 * Correct `select` style inheritance in Firefox 4+ and Opera.
 */

button,
select {
    text-transform: none;
}

/**
 * 1. Avoid the WebKit bug in Android 4.0.* where (2) destroys native `audio`
 *    and `video` controls.
 * 2. Correct inability to style clickable `input` types in iOS.
 * 3. Improve usability and consistency of cursor style between image-type
 *    `input` and others.
 */

button,
html input[type="button"], /* 1 */
input[type="reset"],
input[type="submit"] {
    -webkit-appearance: button; /* 2 */
    cursor: pointer; /* 3 */
}

/**
 * Re-set default cursor for disabled elements.
 */

button[disabled],
html input[disabled] {
    cursor: default;
}

/**
 * 1. Address box sizing set to `content-box` in IE 8/9/10.
 * 2. Remove excess padding in IE 8/9/10.
 */

input[type="checkbox"],
input[type="radio"] {
    box-sizing: border-box; /* 1 */
    padding: 0; /* 2 */
}

/**
 * 1. Address `appearance` set to `searchfield` in Safari 5 and Chrome.
 * 2. Address `box-sizing` set to `border-box` in Safari 5 and Chrome
 *    (include `-moz` to future-proof).
 */

input[type="search"] {
    -webkit-appearance: textfield; /* 1 */
    -moz-box-sizing: content-box;
    -webkit-box-sizing: content-box; /* 2 */
    box-sizing: content-box;
}

/**
 * Remove inner padding and search cancel button in Safari 5 and Chrome
 * on OS X.
 */

input[type="search"]::-webkit-search-cancel-button,
input[type="search"]::-webkit-search-decoration {
    -webkit-appearance: none;
}

/**
 * Remove inner padding and border in Firefox 4+.
 */

button::-moz-focus-inner,
input::-moz-focus-inner {
    border: 0;
    padding: 0;
}

/**
 * 1. Remove default vertical scrollbar in IE 8/9.
 * 2. Improve readability and alignment in all browsers.
 */

textarea {
    overflow: auto; /* 1 */
    vertical-align: top; /* 2 */
}

/* ==========================================================================
   Tables
   ========================================================================== */

/**
 * Remove most spacing between table cells.
 */

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

.go-top {
position: fixed;
bottom: 2em;
right: 2em;
text-decoration: none;
background-color: #E0E0E0;
font-size: 12px;
padding: 1em;
display: inline;
}

/* Github css */

html,body{        margin: auto;
    padding-right: 1em;
    padding-left: 1em;
    max-width: 44em; color:black;}*:not('#mkdbuttons'){margin:0;padding:0}body{font:13.34px helvetica,arial,freesans,clean,sans-serif;-webkit-font-smoothing:subpixel-antialiased;line-height:1.4;padding:3px;background:#fff;border-radius:3px;-moz-border-radius:3px;-webkit-border-radius:3px}p{margin:1em 0}a{color:#4183c4;text-decoration:none}body{background-color:#fff;padding:30px;margin:15px;font-size:14px;line-height:1.6}body>*:first-child{margin-top:0!important}body>*:last-child{margin-bottom:0!important}@media screen{body{box-shadow:0 0 0 1px #cacaca,0 0 0 4px #eee}}h1,h2,h3,h4,h5,h6{margin:20px 0 10px;padding:0;font-weight:bold;-webkit-font-smoothing:subpixel-antialiased;cursor:text}h1{font-size:28px;color:#000}h2{font-size:24px;border-bottom:1px solid #ccc;color:#000}h3{font-size:18px;color:#333}h4{font-size:16px;color:#333}h5{font-size:14px;color:#333}h6{color:#777;font-size:14px}p,blockquote,table,pre{margin:15px 0}ul{padding-left:30px}ol{padding-left:30px}ol li ul:first-of-type{margin-top:0}hr{background:transparent url(data:image/png;base64,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) repeat-x 0 0;border:0 none;color:#ccc;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,ul li>:first-child,ol li>: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{border-collapse:collapse;border-spacing:0;font-size:100%;font:inherit}table th{font-weight:bold;border:1px solid #ccc;padding:6px 13px}table td{border:1px solid #ccc;padding:6px 13px}table tr{border-top:1px solid #ccc;background-color:#fff}table tr:nth-child(2n){background-color:#f8f8f8}img{max-width:100%}code,tt{margin:0 2px;padding:0 5px;white-space:nowrap;border:1px solid #eaeaea;background-color:#f8f8f8;border-radius:3px;font-family:Consolas,'Liberation Mono',Courier,monospace;font-size:12px;color:#333}pre>code{margin:0;padding:0;white-space:pre;border:0;background:transparent}.highlight pre{background-color:#f8f8f8;border:1px solid #ccc;font-size:13px;line-height:19px;overflow:auto;padding:6px 10px;border-radius:3px}pre{background-color:#f8f8f8;border:1px solid #ccc;font-size:13px;line-height:19px;overflow:auto;padding:6px 10px;border-radius:3px}pre code,pre tt{background-color:transparent;border:0}.poetry pre{font-family:Georgia,Garamond,serif!important;font-style:italic;font-size:110%!important;line-height:1.6em;display:block;margin-left:1em}.poetry pre code{font-family:Georgia,Garamond,serif!important;word-break:break-all;word-break:break-word;-webkit-hyphens:auto;-moz-hyphens:auto;hyphens:auto;white-space:pre-wrap}sup,sub,a.footnote{font-size:1.4ex;height:0;line-height:1;vertical-align:super;position:relative}sub{vertical-align:sub;top:-1px}@media print{body{background:#fff}img,pre,blockquote,table,figure{page-break-inside:avoid}body{background:#fff;border:0}code{background-color:#fff;color:#333!important;padding:0 .2em;border:1px solid #dedede}pre{background:#fff}pre code{background-color:white!important;overflow:visible}}@media screen{body.inverted{color:#eee!important;border-color:#555;box-shadow:none}.inverted body,.inverted hr .inverted p,.inverted td,.inverted li,.inverted h1,.inverted h2,.inverted h3,.inverted h4,.inverted h5,.inverted h6,.inverted th,.inverted .math,.inverted caption,.inverted dd,.inverted dt,.inverted blockquote{color:#eee!important;border-color:#555;box-shadow:none}.inverted td,.inverted th{background:#333}.inverted h2{border-color:#555}.inverted hr{border-color:#777;border-width:1px!important}::selection{background:rgba(157,193,200,0.5)}h1::selection{background-color:rgba(45,156,208,0.3)}h2::selection{background-color:rgba(90,182,224,0.3)}h3::selection,h4::selection,h5::selection,h6::selection,li::selection,ol::selection{background-color:rgba(133,201,232,0.3)}code::selection{background-color:rgba(0,0,0,0.7);color:#eee}code span::selection{background-color:rgba(0,0,0,0.7)!important;color:#eee!important}a::selection{background-color:rgba(255,230,102,0.2)}.inverted a::selection{background-color:rgba(255,230,102,0.6)}td::selection,th::selection,caption::selection{background-color:rgba(180,237,95,0.5)}.inverted{background:#0b2531;background:#252a2a}.inverted body{background:#252a2a}.inverted a{color:#acd1d5}}.highlight .c{color:#998;font-style:italic}.highlight .err{color:#a61717;background-color:#e3d2d2}.highlight .k,.highlight .o{font-weight:bold}.highlight .cm{color:#998;font-style:italic}.highlight .cp{color:#999;font-weight:bold}.highlight .c1{color:#998;font-style:italic}.highlight .cs{color:#999;font-weight:bold;font-style:italic}.highlight .gd{color:#000;background-color:#fdd}.highlight .gd .x{color:#000;background-color:#faa}.highlight .ge{font-style:italic}.highlight .gr{color:#a00}.highlight .gh{color:#999}.highlight .gi{color:#000;background-color:#dfd}.highlight .gi .x{color:#000;background-color:#afa}.highlight .go{color:#888}.highlight .gp{color:#555}.highlight .gs{font-weight:bold}.highlight .gu{color:#800080;font-weight:bold}.highlight .gt{color:#a00}.highlight .kc,.highlight .kd,.highlight .kn,.highlight .kp,.highlight .kr{font-weight:bold}.highlight .kt{color:#458;font-weight:bold}.highlight .m{color:#099}.highlight .s{color:#d14}.highlight .na{color:#008080}.highlight .nb{color:#0086b3}.highlight .nc{color:#458;font-weight:bold}.highlight .no{color:#008080}.highlight .ni{color:#800080}.highlight .ne,.highlight .nf{color:#900;font-weight:bold}.highlight .nn{color:#555}.highlight .nt{color:#000080}.highlight .nv{color:#008080}.highlight .ow{font-weight:bold}.highlight .w{color:#bbb}.highlight .mf,.highlight .mh,.highlight .mi,.highlight .mo{color:#099}.highlight .sb,.highlight .sc,.highlight .sd,.highlight .s2,.highlight .se,.highlight .sh,.highlight .si,.highlight .sx{color:#d14}.highlight .sr{color:#009926}.highlight .s1{color:#d14}.highlight .ss{color:#990073}.highlight .bp{color:#999}.highlight .vc,.highlight .vg,.highlight .vi{color:#008080}.highlight .il{color:#099}.highlight .gc{color:#999;background-color:#eaf2f5}.type-csharp .highlight .k,.type-csharp .highlight .kt{color:#00F}.type-csharp .highlight .nf{color:#000;font-weight:normal}.type-csharp .highlight .nc{color:#2b91af}.type-csharp .highlight .nn{color:#000}.type-csharp .highlight .s,.type-csharp .highlight .sc{color:#a31515}

In case it's useful to others, you can use overflow-x: scroll to have code samples horizontal-scroll rather than overflowing the column boundaries:

/**
 * Improve readability of pre-formatted text in all browsers.
 */

pre {
    white-space: pre-wrap;
    overflow-x: scroll;
}

Thanks!