style.css

:root {
    --cell-size: 100px;
    --mark-size: calc(var(--cell-size)*0.9);
}

body {
    margin: 0;
}

.board {
    width: 100vw;
    height: 100vh;
    justify-content: center;
    align-content: center;
    display: grid;
    grid-template-columns: repeat(3, auto);
}

.cell {
    width: var(--cell-size);
    height: var(--cell-size);
    border: 1px solid black;
    cursor: pointer;
    display: flex;
    justify-content: center;
    align-items: center;
    position: relative;
}

.cell.x, .cell.o {
    cursor: not-allowed;
}

.board.x .cell:not(.x):not(.o):hover::before, .board.x .cell:not(.x):not(.o):hover::after, .board.o .cell:not(.x):not(.o):hover::before {
    background-color: lightgrey;
}

.cell.x::before, .cell.x::after, .cell.o::before {
    background-color: black;
}

.cell.x::before, .cell.x::after, .board.x .cell:not(.x):not(.o):hover::before, .board.x .cell:not(.x):not(.o):hover::after {
    content: '';
    position: absolute;
    height: calc(var(--mark-size)*0.1);
    width: var(--mark-size);
}

.cell.x::before, .board.x .cell:not(.x):not(.o):hover::before {
    transform: rotate(45deg);
}

.cell.x::after, .board.x .cell:not(.x):not(.o):hover::after {
    transform: rotate(-45deg);
}

.cell.o::before, .cell.o::after, .board.o .cell:not(.x):not(.o):hover::before, .board.o .cell:not(.x):not(.o):hover::after {
    content: '';
    border-radius: 50%;
    position: absolute;
}

.cell.o::before, .board.o .cell:not(.x):not(.o):hover::before {
    width: calc(var(--mark-size)*0.9);
    height: calc(var(--mark-size)*0.9);
}

.cell.o::after, .board.o .cell:not(.x):not(.o):hover::after {
    width: calc(var(--mark-size)*0.7);
    height: calc(var(--mark-size)*0.7);
    background-color: white;
}

.winning-message {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: rgba(0, 0, 0, .9);
    justify-content: center;
    align-items: center;
    color: white;
    font-size: 5rem;
    flex-direction: column;
  }
  
  .winning-message button {
    font-size: 3rem;
    background-color: white;
    border: 1px solid black;
    padding: .25em .5em;
    cursor: pointer;
  }
  
  .winning-message button:hover {
    background-color: black;
    color: white;
    border-color: white;
  }
  
  .winning-message.show {
    display: flex;
  }