body {
    background: #f4f4f4;
    font-family: Arial, sans-serif;
    display: flex;
    flex-direction: column;
    align-items: center;
    margin-top: 40px;
}

.calculator {
    background: #222;
    padding: 24px 18px 18px 18px;
    border-radius: 16px;
    box-shadow: 0 4px 24px rgba(0,0,0,0.2);
    width: 260px;
}

#display {
    width: 100%;
    height: 48px;
    font-size: 1.5em;
    margin-bottom: 16px;
    text-align: right;
    padding: 8px;
    border: none;
    border-radius: 8px;
    background: #fff;
    color: #222;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

button {
    padding: 18px 0;
    font-size: 1.2em;
    border: none;
    border-radius: 8px;
    background: #444;
    color: #fff;
    cursor: pointer;
    transition: background 0.2s;
}

button:hover {
    background: #666;
}

button:active {
    background: #888;
}

button:nth-child(1) {
    background: #e74c3c;
    color: #fff;
}

button:nth-child(15) {
    background: #27ae60;
    color: #fff;
}