style.css

body {
  background-color: #1d1d1d;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  gap: 40px;
}

#board {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  grid-template-rows: repeat(9, 1fr);
  width: 400px;
  height: 400px;
}

.cell {
  border: 1px solid rgb(209, 209, 11);
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: x-large;
  color: white;
}

.start-btn {
  border: none;
  padding: 8px 24px;
  border-radius: 10px;
  background-color: green;
  font-weight: bold;
  color: white;
  letter-spacing: 0.5ch;
  font-size: larger;
  width: fit-content;
}

h1 {
  font-weight: bold;
  color: yellow;
  font-family: sans-serif;
  font-size: xx-large;
}

.board-parent {
  position: relative;
}

.border-div {
  z-index: -1;
  position: absolute;
  display: grid;
  width: 400px;
  height: 400px;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: repeat(3, 1fr);
}

.cellBorder {
  border: 2px solid white;
}

.active{
    visibility: visible;
}

.hidden{
    visibility: hidden;
}