gomoku_server_ui/frontend/src/components/board.css

.board {
  display: flex;
  margin: 10px auto;
  flex-direction: column;
  position: relative;
  height: 375px;
  width: 375px;
  max-width: 600px;
  max-height: 600px;
  background-size: cover;
}

.board-row {
  display: flex;
}

.cell {
  position: relative;
  display: flex;
  justify-content: center;
  align-items: center;
}

.cell::before,
.cell::after {
  content: "";
  position: absolute;
  background: #000;
}

.cell::before {
  left: 50%;
  height: 100%;
  width: 1px;
}

.cell::after {
  top: 50%;
  width: 100%;
  height: 1px;
}

.cell.top::before {
  top: 50%;
}
.cell.bottom::before {
  bottom: 50%;
}
.cell.left::after {
  left: 50%;
}
.cell.right::after {
  right: 50%;
}


.piece {
  width: 70%;
  height: 70%;
  border-radius: 50%;
  position: relative;
  z-index: 10;
  font-size: 13px;
  display: flex;
  justify-content: center;
  align-items: center;
}

.piece.black {
  background: #000;
  color: white;
}

.piece.white {
  background: #fff;
  color: black;
}

.last {
  width: 80%;
  height: 80%;
  border: 1px solid red;
  z-index: 100;
  position: absolute;
}