body {
  height: 100vh;
  width: 100vw;
  background-image: url("../img/space.jpg");
  background-size: cover;
  color: #fff;
  font-family: sans-serif;
}

h1 {
  font-family: 'Audiowide', cursive;
  text-align: center;
  font-size: 4em;
}

#board {
  height: 60%;
  width: 40%;
  margin: 0 auto;
  border: 1px solid #707070;
  background-color: transparent;
  /*padding: 5px;*/
}

.cell {
  min-height: 25%;
  min-width: 20%;
  background-color: rgba(255,255,255,0.25);
  display: inline-block;
  margin: 0.25%;
.left, .middle-left {
  border-right: 10px solid #fff;
}

.right {
  border-left: 10px solid #fff;
}

p {
  text-align: center;
  font-size: 4em;
  padding-top: 10px;
  margin: 0 auto;
}
body {
  height: 100vh;
  width: 100vw;
  background-image: url("../img/space.jpg");
  background-size: cover;
  color: #fff;
  font-family: sans-serif;
}

h1 {
  font-family: 'Audiowide', cursive;
  text-align: center;
  font-size: 4em;
}

#board {
  height: 60%;
  width: 40%;
  margin: 0 auto;
  border: 1px solid #707070;
  background-color: transparent;
  /*padding: 5px;*/
}

.cell {
  min-height: 25%;
  min-width: 20%;
  background-color: rgba(255,255,255,0.25);
  display: inline-block;
  margin: 0.25%;
#first, #second, #third {
  border-bottom: 10px solid #fff;
}

.left, .middle-left {
  border-right: 10px solid #fff;
}

.right {
  border-left: 10px solid #fff;
}

p {
  text-align: center;
  font-size: 4em;
  padding-top: 10px;
  margin: 0 auto;
}
Add bottom border to first second and third divs
Add bottom border to first second and third divs
