It's more CSS-like than actual CSS.

gistfile1.css

/*
  h1 through to h6 map to the Markdown heading attributes, p matches any paragraph, there
  isn't any ID or class support (yet!).

  All sizes should be in percentages or points. Everything is calculated relative to the page's
  font-size.
*/

page {
  page-size: 'A4';
  page-orientation: landscape;
  font-size: 12pt;
  color: #000;
}

h1 {
  font-face: 'Trebuchet MS', sans-serif;
  font-size: 150%;
  font-weight: bold;
}

p {
  padding: 10pt;
}