* {
    box-sizing: border-box;
}

body {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    height: 100vh;
   
    padding: 0;
    font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
    background-color: rgb(80, 124, 225);
}

.container {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
}

.memo-box {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-template-rows: repeat(4, 1fr);
    gap: 10px; 
    margin: 20px 20px  20px  20px;
    border-radius: 10px;
    height: 400px;
    width: 400px;
    background-color: white;
    padding: 10px 10px 0px 10px;
    perspective: 1000px;
}

.cell {
    display: flex;
    justify-content: center;
    align-items: center;
    border-radius: 5px;
    background-color: lightgray;
    font-size: 4rem;
    cursor: pointer;
    position: relative;
    perspective: 1000px;
}

.card {
    width: 100%;
    height: 100%;
    position: absolute;
    transition: transform 0.6s;
    transform-style: preserve-3d;
}

.cell.show .card {
    transform: rotateY(180deg);
}

.front, .back {
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    position: absolute;
    display: flex;
    justify-content: center;
    align-items: center;
}

.front {
    background-color: #fff;
    transform: rotateY(180deg);
}

.back {
    background-color: lightgray;
}