body {
  box-sizing: border-box;
  min-height: 100vh;
  margin: 0 auto;
  width: 900px;
  background: black;
  color: white;
  padding: 20px 30px;
  text-align: center;
}
#image {
  box-sizing: border-box;
  width: 800px;
  height: 500px;
  margin: 40px auto;
  background: white url("bilder/bild1.jpg") no-repeat center center;
  background-size: cover;
  transition: background 0.5s;
  border: 1px solid black;
  position: relative;
}
#thumbs img {
  width: 160px;
  height: 100px;
  border: 1px solid black;
  margin: 5px;
  display: inline-block;
  filter: grayscale(100%);
}
#thumbs img:hover {
  cursor: pointer;
  filter: grayscale(0);
}
#left {
  width: 20%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  display: flex;
  background: rgba(255, 255, 255, 0);
}
#right {
  width: 20%;
  height: 100%;
  position: absolute;
  top: 0;
  right: 0;
  display: flex;
  background: rgba(255, 255, 255, 0);
}
#left:hover,
#right:hover {
  display: flex;
  cursor: pointer;
  background: rgba(255, 255, 255, 0.5);
}
#image span {
  font-size: 60px;
  display: none;
  margin: auto;
}
#left:hover span,
#right:hover span {
  display: block;
}