styles.css

.grid-container {
    width: 100vw;
    height: 100vh;
    display: grid;
    grid-template-columns: auto auto auto;
    justify-content: center;
    align-content: center;
}

.cell {
    width: 100px;
    height: 100px;
    border: 1px solid black;
    font-size: 100px;
    display: flex;
    justify-content: center;
}

.cell:first-child,
.cell:nth-child(2),
.cell:nth-child(3)
 {
    border-top: none;
}

.cell:first-child,
.cell:nth-child(4),
.cell:nth-child(7)
 {
    border-left: none;
}

.cell:nth-child(3),
.cell:nth-child(6),
.cell:nth-child(9)
 {
    border-right: none;
}

.cell:nth-child(7),
.cell:nth-child(8),
.cell:nth-child(9)
 {
    border-bottom: none;
}

.cell.x::before {
    content: 'X';
}

.cell.o::before {
    content: 'O';
}

.grid-container.x .cell:not(.x):not(.o):hover::before {
    color: lightgray;
    content: 'X';
}

.grid-container.o .cell:not(.x):not(.o):hover::before {
    color: lightgray;
    content: 'O';
}

.winning-message {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: rgba(0, 0, 0, 0.9);    
    justify-content: center;
    align-items: center;
    color: white;
    font-size: 3rem;
}

.winning-message.show {
    display: flex;
}