main.css

h1 {
    text-align: center;
    font-size: 40px;
    font-family: Arial, Helvetica, sans-serif;
    font-weight: 700;
}


.area-wrapper {
    width: 100%;
    height: 100%;
    display: flex;
    justify-content: center;
    align-items: center;
}

#area {
    width: 300px;
    height: 300px;
    background-color: rgb(192, 192, 192);
    display: flex;
    flex-wrap: wrap;
    border: 1px solid black;
}

.box {
    width: 100px;
    height: 100px;
    border: 1px solid black;
    box-sizing: border-box;
    font-size: 34px;
    color: black;
    display: flex;
    justify-content: center;
    align-items: center;
}

#modal-result-wrapper {
    display: none;
    width: 100%;
    height: 100%;
    font-family: Arial, Helvetica, sans-serif;
    font-weight: 600;
    top: 0;
    left: 0;
}
#overlay {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
     background-color: rgba(0, 0, 0, 0.808);
}

#modal-window {
    width: 150px;
    height: 100px;
    border: 3px solid black;
    background-color: white;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate( -50%, -50%);
    display: flex;
    flex-flow: column;
    justify-content: space-between;
    align-items: center;
    text-align: center;
    padding-top: 20px;
}

#btn-close {
    width: 150px;
    height: 30px;
    background-color: rgba(158, 158, 158, 0.979);
    cursor: pointer;
    border-top: 3px solid black;
    display: flex;
    align-items: center;
    justify-content: center;
}