body {
  font-family: sans-serif;
  text-align: center;
}
#puzzle-board {
  margin: 20px auto;
  display: flex;
  flex-wrap: wrap;
  width: 300px;
  height: 300px;
  border: 2px solid #000;
}
.tile {
  box-sizing: border-box;
  border: 1px solid #ccc;
  background-size: 300px 300px;
}
.empty {
  background: #eee;
}
.modal {
  display: none;
  position: fixed;
  z-index: 999;
  left: 0; top: 0;
  width: 100%; height: 100%;
  background-color: rgba(0,0,0,0.6);
}
.modal-content {
  background: #fff;
  margin: auto;
  padding: 20px;
  width: 90%;
  max-width: 500px;
  border-radius: 8px;
}
.close {
  float: right;
  font-size: 24px;
  cursor: pointer;
}