style.css

html, body {
    margin: 0;
    padding: 0;
    overflow: hidden;
}

.body {
    display: flex;
    flex-direction: column;
    align-items: center;
}

.heading {
    margin: 20px auto;
    width: fit-content;
    font-size: 24px;
    font-weight: bold;
    color: blue;
}

#message-box {
    display: flex;
    justify-content: center;
    align-items: center;
    margin: 20px auto;
    width: 300px;
    height: 80px;
    border: 2px solid black;
    border-radius: 10px;
    color: green;
    text-align: center;
}

#calculator {
    display: flex;
    margin: 40px auto;
    padding: 8px;
    background-color: paleturquoise;
    border: 2px solid black;
    border-radius: 10px;
    flex-direction: column;
    gap: 2px;
    height: 480px;
    width: 330px;
}

@media screen and (min-width: 1100px) {
    .body {
        flex-direction: row;
        justify-content: center;
        justify-items: center;
        gap: 200px;
    }

    #message-box {
        margin: 20px 0;
    }

    #calculator {
        margin: 40px 0;
    }
}

#display {
    display: flex;
    background-color: white;
    justify-content: end;
    align-items: center;
    padding: 4px;
    height: 50px;
    text-align: right;
    border: 1px solid black;
}

.container {
    display: flex;
    gap: 2px;
    flex: 1 0 0;
}

.btn {
    flex: 1 0 0;
    font-size: 20px;
}

#clear {
    flex: 1 0 0;
}