div.thumbnails {
  width: 400px;
  display: block;
  position: relative;
  margin: auto;
}
div.gutter{
  position: relative;
}

.gutter-images img{
  width: 20%;
  margin: 0;
  padding: 0;
  display: none;
  float: left;
  height: 70px;
}

.active img {
  width: 100%;
  height: 400px;
}

.gutter-images img.in-gutter {
  display: block;
}

a.nav {
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
  width: 5%;
  font-size: 36px;
}

a.nav.right{
  left: 95%;
}

.gutter-images {
  width: 90%;
  position: relative;
  left: 5%;
}

.group::after {
  display: block;
  clear: both;
  content: "";
}