.viewer-invert
{
  display: flex !important;
  align-items: center;
  justify-content: center;
}

.viewer-invert::before
{
  content: '';
  display: inline-block;
  width: 18px;
  height: 18px;

  background-color: #3F3F3F;

  background-image: url("../images/viewerjs-invert-icon.png");
  background-size: contain;
  background-repeat: no-repeat;
  background-position: center;
}