.islandora-openseadragon {
  display: block;
  width: 100%;
  height: 500px;
}
.islandora-openseadragon canvas {
  background-color: #000 !important; /* should be replaced with OpenSeadragon setting */
}

.islandora-openseadragon-highlight {
  background-color: #f00;
  opacity: 0.5;
}