github-monospace.css

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain('github.com') {
  div.file div.input *, div.file div.name *, div.meta div.info *, div.meta div.actions *, .url-field, div#browser *, .machine, tt, pre, kbd, code {
    font-family: "DejaVu Sans Mono", "Monaco", "Consolas", "Andale Mono", monospace !important;
  }
}

github-twilight.css

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("github.com") {
  /* The Source Code area: */
  #files .file .data {
    background-color: #141414 !important;
    color:#F8F8F8 !important;
  }

  /* Color Definitions: */

  /* Comments */
  .syntax .sd {
    color:#5F5A60 !important;
    font-style:italic !important;
  }

  /* Class names and stuff (class, public, function, etc) */
  .syntax .k {
    color:#F9EE98 !important;
    font-weight:normal !important;
  }

  /* Function Names */
  .syntax .nb {
    color:#DAD085 !important;
  }

  /* Class Names */
  .syntax .nc {
    color:#9B703F !important;
    font-weight:bold !important;
  }

  /* Variables */
  .syntax .nv {
    color:#7587A6 !important;
  }

  /* Class variables */
  .syntax .na {
    color:#7587A6 !important;
  }

  /* Operators ('->', '::', '>', '<', '=', etc ) */
  .syntax .o {
    color:#CDA869 !important;
  }

  /* Strings */
  .syntax .s1 {
    color:#8F9D6A !important;
  }

  /* Constants */
  .syntax .o + .k,
  .syntax .nx {
    color:#CF6A4C !important;
  }
}