style.css

* {
    box-sizing: border-box;
}
body {
    background-color: #000000;
}
.container {
    max-width: 500px;
    padding: 0 30px;
    margin: 100px auto;
}
.field {
    margin-bottom: 20px;
}
.field label {
    color: #1f1f1f;
}
#board {
    width: 100%;
    padding-top: 100%;
    position: relative;
    margin-bottom: 30px;
}
#board .cells-wrap {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: flex;
    flex-direction: row;
    flex-wrap: wrap;
}
#board [class^="cell-"] {
    height: 33.3333333%;
    width: 33.3333333%;
    border: 2px solid #0a0710;
    background: #e0e0e0;
    position: relative;
    cursor: pointer;
    color: #292929;
    font-size: calc(18px + 5vw);
    font-family: sans-serif;
}

#board [class^="cell-"].x, #board [class^="cell-"].o {
    cursor: not-allowed;
}
#board [class^="cell-"].x:after {
    content: 'x';
}
#board [class^="cell-"].o:after {
    content: 'o';
}

#board:after {
    content: '';
    position: absolute;
    background-color: #c11dd4; 
    transition: 0.7s;
}

/* Horizontal Lines */
#board[class^="h-"]:after {
    width: 0%;
    height: 3px;
    left: 0;
    transform: width translateY(-50%);
}
#board.fullLine[class^="h-"]:after {
    width: 100%;
}
#board.h-1:after {
    top: 16.6666666665%;
}
#board.h-2:after {
    top: 50%;
}
#board.h-3:after {
    top: 83.33333333%;
}

/* Vertical Lines */
#board[class^="v-"]:after {
    width: 3px;
    height: 0%;
    top: 0;
    transform: height translateX(-50%);
}
#board.fullLine[class^="v-"]:after {
    height: 100%;
}
#board.v-1:after {
    left: 16.6666666665%;
}
#board.v-2:after {
    left: 50%;
}
#board.v-3:after {
    left: 83.33333333%;
}

/* Diagonal Lines */
#board[class^="d-main"]:after {
    width: 3px;
    height: 0%;
    left: 0;
    top: 0;
    transform: rotateZ(-45deg);
    transform-origin: 50% 0;
    transition: height 0.7s;
}
#board.fullLine[class^="d-main"]:after {
    height: 140%;
}

#board[class^="d-counter"]:after {
    height: 0%;
    width: 3px;
    right: 0;
    top: 0;
    transform: rotateZ(45deg);
    transform-origin: 50% 0;
    transition: height 0.7s;
}
#board.fullLine[class^="d-counter"]:after {
    height: 140%;
}