silverstripe-contentreview/client/src/styles/ContentReviewForm.scss

// The bell button, shows up next to the major actions (save, publish, etc) when
// viewing a page in the CMS
.content-review__button {
  background: url('../images/icon-bell.png') center center no-repeat;
  background-position: 0 0;
  display: inline-block;
  height: 20px;
  margin: 6px 4px 0 12px;
  padding: 0;
  text-indent: -9999px;
  width: 20px;

  &:hover,
  &:focus {
    background-position: 0 -20px;
  }
}