body {
  height: 100vh;
  width: 100vw;
  background-image: url("../img/space.jpg");
  background-size: cover;
  color: #fff;
  font-family: sans-serif;
}
#board {
  height: 400px;
  width: 800px;
  background-color: #3399ff;
  padding: 5px;
}

.cell {
  height: 94px;
  width: 191px;
  background-color: #ffcc33;
  display: inline-block;
  margin: 3px;
}

p {
  text-align: center;
  font-size: 4em;
  margin: 0 auto;
}
body {
  height: 100vh;
  width: 100vw;
  background-image: url("../img/space.jpg");
  background-size: cover;
  color: #fff;
  font-family: sans-serif;
}
#board {
  height: 400px;
  width: 800px;
  background-color: #3399ff;
  padding: 5px;
}

.cell {
  min-height: 25%;
  min-width: 20%;
  background-color: rgba(255,255,255,0.25);
  display: inline-block;
  margin: 0.25%;
}

p {
  text-align: center;
  font-size: 4em;
  margin: 0 auto;
}
Change cell dimensions to percentage of board container, transparent white background, and 0.25% margin
Change cell dimensions to percentage of board container, transparent white background, and 0.25% margin
