gh-fork-ribbon.css

/* Left will inherit from right (so we don't need to duplicate code) */
.github-fork-ribbon {
  /* The right and left classes determine the side we attach our banner to */
  position: absolute;

  /* Add a bit of padding to give some substance outside the "stitching" */
  padding: 2px 0;

  /* Set the base colour */
  background-color: #a00;

  /* Set a gradient: transparent black at the top to almost-transparent black at the bottom */
  background-image: -webkit-gradient(linear, left top, left bottom, from(rgba(0, 0, 0, 0)), to(rgba(0, 0, 0, 0.15)));
  background-image: -webkit-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.15));
  background-image: -moz-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.15));
  background-image: -ms-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.15));
  background-image: -o-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.15));
  background-image: linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.15));

  /* Add a drop shadow */
  -webkit-box-shadow: 0 2px 3px 0 rgba(0, 0, 0, 0.5);
  -moz-box-shadow: 0 2px 3px 0 rgba(0, 0, 0, 0.5);
  box-shadow: 0 2px 3px 0 rgba(0, 0, 0, 0.5);

  z-index: 9999;
  pointer-events: auto;
}

.github-fork-ribbon a,
.github-fork-ribbon a:hover {
  /* Set the font */
  font: 700 13px "Helvetica Neue", Helvetica, Arial, sans-serif;
  color: #fff;

  /* Set the text properties */
  text-decoration: none;
  text-shadow: 0 -1px rgba(0, 0, 0, 0.5);
  text-align: center;

  /* Set the geometry. If you fiddle with these you'll also need
     to tweak the top and right values in .github-fork-ribbon. */
  width: 200px;
  line-height: 20px;

  /* Set the layout properties */
  display: inline-block;
  padding: 2px 0;

  /* Add "stitching" effect */
  border-width: 1px 0;
  border-style: dotted;
  border-color: #fff;
  border-color: rgba(255, 255, 255, 0.7);
}

.github-fork-ribbon-wrapper {
  width: 150px;
  height: 150px;
  position: absolute;
  overflow: hidden;
  top: 0;
  z-index: 9999;
  pointer-events: none;
}

.github-fork-ribbon-wrapper.fixed {
  position: fixed;
}

.github-fork-ribbon-wrapper.left {
  left: 0;
}

.github-fork-ribbon-wrapper.right {
  right: 0;
}

.github-fork-ribbon-wrapper.left-bottom {
  position: fixed;
  top: inherit;
  bottom: 0;
  left: 0;
}

.github-fork-ribbon-wrapper.right-bottom {
  position: fixed;
  top: inherit;
  bottom: 0;
  right: 0;
}

.github-fork-ribbon-wrapper.right .github-fork-ribbon {
  top: 42px;
  right: -43px;

  -webkit-transform: rotate(45deg);
  -moz-transform: rotate(45deg);
  -ms-transform: rotate(45deg);
  -o-transform: rotate(45deg);
  transform: rotate(45deg);
}

.github-fork-ribbon-wrapper.left .github-fork-ribbon {
  top: 42px;
  left: -43px;

  -webkit-transform: rotate(-45deg);
  -moz-transform: rotate(-45deg);
  -ms-transform: rotate(-45deg);
  -o-transform: rotate(-45deg);
  transform: rotate(-45deg);
}


.github-fork-ribbon-wrapper.left-bottom .github-fork-ribbon {
  top: 80px;
  left: -43px;

  -webkit-transform: rotate(45deg);
  -moz-transform: rotate(45deg);
  -ms-transform: rotate(45deg);
  -o-transform: rotate(45deg);
  transform: rotate(45deg);
}

.github-fork-ribbon-wrapper.right-bottom .github-fork-ribbon {
  top: 80px;
  right: -43px;

  -webkit-transform: rotate(-45deg);
  -moz-transform: rotate(-45deg);
  -ms-transform: rotate(-45deg);
  -o-transform: rotate(-45deg);
  transform: rotate(-45deg);
}