header {
  text-align: center;
}

#gameboard {
  width: 100vw;
  height: 50vh;
  display: grid;
  justify-content: center;
  align-content: center;
  justify-items: center;
  align-items: center;
  grid-template-columns: repeat(3, auto);
}

.cell {
  width: 100px;
  height: 100px;
  border: 1px solid black;
  display: flex;
  justify-content: center;
  align-items: center;
  position: relative;
  cursor: pointer;
  font-size: 50px;
}

.cell:first-child,
.cell:nth-child(2),
.cell:nth-child(3) {
  border-top: none;
}

.cell:nth-child(3n + 1) {
  border-left: none;
}

.cell:nth-child(3n + 3) {
  border-right: none;
}

.cell:last-child,
.cell:nth-child(7),
.cell:nth-child(8) {
  border-bottom: none;
}

#controls, #message {
  text-align: center;
  margin: 20px 0;
}