* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Segoe UI', sans-serif;
}

body {
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background: #1f1f1f;
}

.calculator {
    width: 320px;
    background: #2c2c2c;
    border-radius: 15px;
    overflow: hidden;
    box-shadow: 0 10px 30px rgba(0,0,0,0.4);
}

.display {
    background: #000;
    color: #00ffcc;
    font-size: 2.5rem;
    padding: 20px;
    text-align: right;
}

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

button {
    padding: 20px;
    font-size: 1.2rem;
    border: none;
    outline: none;
    cursor: pointer;
    background: #3a3a3a;
    color: white;
    transition: background 0.2s;
}

button:hover {
    background: #4a4a4a;
}

.operator {
    background: #ff9500;
    color: white;
}

.operator:hover {
    background: #e08900;
}

.equals {
    background: #00c896;
    color: white;
}

.equals:hover {
    background: #00a67e;
}

.zero {
    grid-column: span 2;
}