body {
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: #1e1e1e;
    font-family: Arial, sans-serif;
}

#calculator {
    background-color: #333;
    border-radius: 15px;
    max-width: 400px;
    width: 90%;
    padding: 20px;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.5);
}

#display {
    width: 100%;
    padding: 15px;
    font-size: 2.5rem;
    text-align: right;
    border: none;
    background-color: #222;
    color: white;
    border-radius: 8px;
    outline: none;
}

#keys {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    margin-top: 20px;
}

button {
    width: 100%;
    height: 70px;
    border-radius: 10px;
    border: none;
    background-color: #444;
    color: white;
    font-size: 2rem;
    font-weight: bold;
    cursor: pointer;
    transition: background 0.3s, transform 0.1s;
}

button:hover {
    background-color: #666;
}

button:active {
    background-color: #888;
    transform: scale(0.95);
}

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

.operator-btn:hover {
    background-color: #ffa733;
}

.operator-btn:active {
    background-color: #ffbe5c;
}

@media (max-width: 500px) {
    button {
        height: 60px;
        font-size: 1.5rem;
    }
    #display {
        font-size: 2rem;
        padding: 10px;
    }
}