how to set page/document margins with css when generating pdf from mkd via pandoc&wkhtmltopdf?

I have this script

#!/usr/bin/env bash
set -euxo pipefail
MARGIN=.35in
pandoc -t html5 -V margin-top=$MARGIN -V margin-left=$MARGIN -V margin-bottom=$MARGIN -V margin-right=$MARGIN -V papersize=letter --css ~/bin/inc/pandoc-pdf.css $1 -o $2 -s --pdf-engine=wkhtmltopdf

which nicely generates a pdf file from a markdown file...

but what's not nice is that I'm setting the MARGIN manually via commandline parameter since I couldn't get the css to do it :/

In addition to fussing endlessly over the css I've tried using alternate pandoc templates (my guessing attempts with those templates don't feel worth sharing, I'm just using pandoc's default html template). The CSS looks like:

/*
 * I add this to html files generated with pandoc.
 */

html {
  font-size: 100%;
  overflow-y: scroll;
  -webkit-text-size-adjust: 100%;
  -ms-text-size-adjust: 100%;
}

body {
  color: #444;
  font-family: Georgia, Palatino, 'Palatino Linotype', Times, 'Times New Roman', serif;
  font-size: 12px;
  line-height: 1.7;
  padding: 0;
  margin: auto;
  max-width: 32em;
  background: #fefefe;
}

a {
  color: #0645ad;
  text-decoration: none;
}

a:visited {
  color: #0b0080;
}

a:hover {
  color: #06e;
}

a:active {
  color: #faa700;
}

a:focus {
  outline: thin dotted;
}

*::-moz-selection {
  background: rgba(255, 255, 0, 0.3);
  color: #000;
}

*::selection {
  background: rgba(255, 255, 0, 0.3);
  color: #000;
}

a::-moz-selection {
  background: rgba(255, 255, 0, 0.3);
  color: #0645ad;
}

a::selection {
  background: rgba(255, 255, 0, 0.3);
  color: #0645ad;
}

p {
  margin: 1em 0;
}

img {
  max-width: 100%;
}

h1, h2, h3, h4, h5, h6 {
  color: #111;
  line-height: 125%;
  margin-top: 2em;
  font-weight: normal;
}

h4, h5, h6 {
  font-weight: bold;
}

h1 {
  font-size: 2.5em;
}

h2 {
  font-size: 2em;
}

h3 {
  font-size: 1.5em;
}

h4 {
  font-size: 1.2em;
}

h5 {
  font-size: 1em;
}

h6 {
  font-size: 0.9em;
}

blockquote {
  color: #666666;
  margin: 0;
  padding-left: 3em;
  border-left: 0.5em #EEE solid;
}

hr {
  display: block;
  height: 2px;
  border: 0;
  border-top: 1px solid #aaa;
  border-bottom: 1px solid #eee;
  margin: 1em 0;
  padding: 0;
}

pre, code, kbd, samp {
  color: #000;
  font-family: monospace, monospace;
  _font-family: 'courier new', monospace;
  font-size: 0.98em;
}

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

b, strong {
  font-weight: bold;
}

dfn {
  font-style: italic;
}

ins {
  background: #ff9;
  color: #000;
  text-decoration: none;
}

mark {
  background: #ff0;
  color: #000;
  font-style: italic;
  font-weight: bold;
}

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

sup {
  top: -0.5em;
}

sub {
  bottom: -0.25em;
}

ul, ol {
  margin: 1em 0;
  padding: 0 0 0 2em;
}

li p:last-child {
  margin-bottom: 0;
}

ul ul, ol ol {
  margin: .3em 0;
}

dl {
  margin-bottom: 1em;
}

dt {
  font-weight: bold;
  margin-bottom: .8em;
}

dd {
  margin: 0 0 .8em 2em;
}

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

img {
  border: 0;
  -ms-interpolation-mode: bicubic;
  vertical-align: middle;
}

figure {
  display: block;
  text-align: center;
  margin: 1em 0;
}

figure img {
  border: none;
  margin: 0 auto;
}

figcaption {
  font-size: 0.8em;
  font-style: italic;
  margin: 0 0 .8em;
}

table {
  margin-bottom: 2em;
  border-bottom: 1px solid #ddd;
  border-right: 1px solid #ddd;
  border-spacing: 0;
  border-collapse: collapse;
}

table th {
  padding: .2em 1em;
  background-color: #eee;
  border-top: 1px solid #ddd;
  border-left: 1px solid #ddd;
}

table td {
  padding: .2em 1em;
  border-top: 1px solid #ddd;
  border-left: 1px solid #ddd;
  vertical-align: top;
}

.author {
  font-size: 1.2em;
  text-align: center;
}

@media only screen and (min-width: 480px) {
  body {
    font-size: 14px;
  }
}
@media only screen and (min-width: 768px) {
  body {
    font-size: 16px;
  }
}
@media print {
  * {
    background: transparent !important;
    color: black !important;
    filter: none !important;
    -ms-filter: none !important;
  }

  body {
    font-size: 12pt;
    max-width: 100%;
    margin: 1em;
    margin-top: 1em !important;
  }

  a, a:visited {
    text-decoration: underline;
  }

  hr {
    height: 1px;
    border: 0;
    border-bottom: 1px solid black;
  }

  a[href]:after {
    content: " (" attr(href) ")";
  }

  abbr[title]:after {
    content: " (" attr(title) ")";
  }

  .ir a:after, a[href^="javascript:"]:after, a[href^="#"]:after {
    content: "";
  }

  pre, blockquote {
    border: 1px solid #999;
    padding-right: 1em;
    page-break-inside: avoid;
  }

  tr, img {
    page-break-inside: avoid;
  }

  img {
    max-width: 100% !important;
  }

  @page :left {
    margin: 35mm 35mm 35mm 35mm;
  }

  @page :right {
    margin: 35mm 35mm 35mm 35mm;
  }

  p, h2, h3 {
    orphans: 3;
    widows: 3;
  }

  h2, h3 {
    page-break-after: avoid;
  }
}

What can I do with CSS to avoid setting MARGIN via commandline parameter?