tic.css

:root {
    --cell-size: 100px;
    --mark-size: calc(var(--cell-size) * .9);
}

*, *::before, *::after {
    box-sizing: border-box;
    outline: none;
    list-style: none;
    border: none;
    text-decoration: none;
    font-family: sans-serif;
    padding: 0%; margin: 0%;
    background: none;
}
button {
    cursor: pointer;
}
button:active {
    opacity: 0.9;
}

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

.box {
    width: var(--cell-size);
    height: var(--cell-size);
    border: 1px solid #000;
    display: flex;
    align-items: center;
    position: relative;
    cursor: pointer;
    justify-content: center;
}

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

.box:nth-child(3n + 1) {
    border-left: none;
}

.box:nth-child(3n + 3) {
    border-right: none;
}

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

.box.x::after,
.box.x::before,
.game.x .box:not(.x):not(.circle):hover:before,
.game.x .box:not(.x):not(.circle):hover:after {
    content: '';
    position: absolute;
    width: calc(var(--mark-size) * .15);
    height: var(--mark-size);
    background: #000;
}

.game.x .box:not(.x):not(.circle):hover::after,
.game.x .box:not(.x):not(.circle):hover::before {
    background: lightgray;
}

.box.x::before,
.game.x .box:not(.x):not(.circle):hover::before {
    transform: rotate(45deg);
}
.box.x::after,
.game.x .box:not(.x):not(.circle):hover:after {
    transform: rotate(-45deg);
}


.box.circle::after,
.box.circle::before,
.game.circle .box:not(.x):not(.circle):hover::before,
.game.circle .box:not(.x):not(.circle):hover::after {
    content: '';
    position: absolute;
    border-radius: 50%;
    background: #000;
}

.box.circle::before,
.game.circle .box:not(.x):not(.circle):hover::before {
    width: var(--mark-size);
    height: var(--mark-size);
    background: #000;
}

.box.circle::after,
.game.circle .box:not(.x):not(.circle):hover::after {
    width: calc(var(--mark-size) * .7);
    height: calc(var(--mark-size) * .7);
    background: #FFF;
}

.game.circle .box:not(.x):not(.circle):hover:before {
    background: lightgray;
}

.winning-message {
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0%;
    background: rgba(0, 0, 0, .9);
    display: none;
    align-items: center;
    justify-content: center;
    flex-direction: column;
    color: #FFF;
    font-size: 4em;
}

#restartButton {
    margin-top: 1em;
    color: #FFF;
}

.show {
    display: flex;
}

.box {
    font-size: 3em;
}