body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: #f0f2f5;
}

#calculator {
    border: 1px solid #000;
    padding: 20px;
    background-color: #fff;
}

#display {
    width: calc(4 * 40px + 3 * 10px); /* Four buttons wide, accounting for margins */
    margin-bottom: 10px;
    padding: 10px;
    text-align: right;
}

.button-row {
    display: flex;
    justify-content: space-between;
    margin-bottom: 10px;
}

#calculator button {
    width: 40px;
    height: 40px;
    margin: 0 5px;
    font-size: 1.2rem;
    cursor: pointer;
}

#calculator .button-row:last-child {
    margin-bottom: 0;
}