body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background: #e0e0e0;
    font-family: Arial, sans-serif;
}

.calculator {
    width: 280px;
    background: #222;
    padding: 20px;
    border-radius: 12px;
}

.display {
    height: 60px;
    background: #000;
    color: #00ff99;
    font-size: 28px;
    text-align: right;
    padding: 10px;
    border-radius: 6px;
    margin-bottom: 15px;
}

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

button {
    height: 55px;
    font-size: 20px;
    border: none;
    border-radius: 6px;
    cursor: pointer;
}

.number {
    background: #444;
    color: #fff;
}

.operator {
    background: #ff9f0a;
    color: #fff;
}

.equal {
    background: #34c759;
    color: #fff;
}

.clear {
    background: #ff3b30;
    color: #fff;
}

.empty {
    visibility: hidden;
}