style.css

/*Extra small devices (phones, 600px down) */
@media only screen and (max-width: 600px) {
    .board {
        width: 405px !important;
        height: 405px !important;
    }
}

/*Extra small devices (phones, 600px down) */
@media only screen and (max-width: 600px) {
    .square{
        width: 135px !important;
        height: 135px !important;
    }
}

@media only screen and (max-width: 425px) {
    .board {
        width: 300px !important;
        height: 400px !important;
    }
}

/*Extra small devices (phones, 425px up) */
@media only screen and (max-width: 425px) {
    .square{
        width: 100px !important;
        height: 135px !important;
    }
}


/* Small devices (portrait tablets and large phones, 768px and up) */
@media only screen and (min-width: 768px) {
    .board{
        width: 600px !important;
        height: 600px !important;
    }
}

/* Small devices (portrait tablets and large phones, 768px and up) */
@media only screen and (min-width: 768px) {
    .square {
        width: 200px !important;
        height: 200px !important;
    }
}

body {
    display: flex;
    justify-content: center;
    align-items: center
}

.board {
    display: flex;
    flex-wrap: wrap;
    flex-direction: row;
    justify-content: flex-start;
    width: 600px;
    height: 600px;
    border: 1px solid black
}

.square {
    width: 200px;
    height: 200px;
    border: 1px solid black;
    box-sizing: border-box;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 6em
}

.square:nth-child(odd){
    background: #FFEB3B;
}

.square:nth-child(even){
    background: #eae1ea
}

.square:hover {
    cursor: pointer;
    background: gray
}