style.css

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: Arial, Helvetica, sans-serif;
}

.container {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  background: #eee;
}

h1 {
  font-size: 5rem;
  margin-bottom: 0.5em;
}

h2 {
  margin-top: 1em;
  font-size: 2rem;
  margin-bottom: 0.5em;
}

.play-area {
  display: grid;
  width: 300px;
  height: 300px;
  grid-template-columns: auto auto auto;
}

.block {
  display: flex;
  width: 100px;
  height: 100px;
  align-items: center;
  justify-content: center;
  font-size: 3rem;
  font-weight: bold;
  border: 3px solid black;
  transition: background 0.2s ease-in-out;
}

.block:hover {
  cursor: pointer;
  background: #0ff30f;
}

.occupied:hover {
  background: #ff3a3a;
}

.win {
  background: #0ff30f;
}

.win:hover {
  background: #0ff30f;
}

#block_0,
#block_1,
#block_2 {
  border-top: none;
}

#block_0,
#block_3,
#block_6 {
  border-left: none;
}

#block_6,
#block_7,
#block_8 {
  border-bottom: none;
}

#block_2,
#block_5,
#block_8 {
  border-right: none;
}

button {
  outline: none;
  border: 4px solid green;
  padding: 10px 20px;
  font-size: 1rem;
  font-weight: bold;
  background: none;
  transition: all 0.2s ease-in-out;
}

button:hover {
  cursor: pointer;
  background: green;
  color: white;
}

.playerWin {
  color: green;
}

.computerWin {
  color: red;
}

.draw {
  color: orangered;
}

@media only screen and (max-width: 600px) {

  h1 {
    font-size: 3rem;
    margin-bottom: 0.5em;
  }

  h2 {
    margin-top: 1em;
    font-size: 1.3rem;
  }
}