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;
}

the nested list seems not work:

1. For this
   1. do this
      1. then this
      2. then this
   2. do this too

it should automatically change the second level to i. 3rd level to a.

1. For this
   1. do this
      1. then this
      2. then this
   2. do this too

but your css does not do this.

I am a bit late to the party.
Is there any update?
Specially for dark mode?
Is it possible to get a dark-mode css for this?

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!