style.css

*, *::after, *::before{
    box-sizing: border-box;
}

:root{
    --cell-size: 100px;
    --mark-size: calc(var(--cell-size)*.7);
}

body{
    margin: 0;
}

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

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

/* remove border from top of the board */
.square:first-child,
.square:nth-child(2),
.square:nth-child(3){
    border-top: none;
}

/* remove border from left side of the board */
.square:nth-child(3n+1){
    border-left: none;
}

/* remove border from right side of the board */
.square:nth-child(3n+3){
    border-right: none;
}

/* remove border from bottom of the board */
.square:last-child,
.square:nth-child(7),
.square:nth-child(8){
    border-bottom: none;
}

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

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

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

/* Creating X element and hovering on X's turn */

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

.square.x::before,
.square.x::after,
.board.x .square:not(.x):not(.o):hover::before,
.board.x .square:not(.x):not(.o):hover::after{
    content: '';
    position: absolute;
    width: calc(var(--mark-size)*.15);
    height: var(--mark-size);
}

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

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

/* Create O element and hovering on O's turn */

.square.o::before{
    background-color: black;
}

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


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

.square.o::after,
.board.o .square:not(.x):not(.o):hover::after{
    width: calc(var(--mark-size)*.7);
    height: calc(var(--mark-size)*.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;
}