:root {
    --green-dark: #0f3d2e;
    --green-light: #2ecc71;
    --green-soft: #1e6f5c;
    --text-light: #eafff5;
}

* {
    box-sizing: border-box;
}

body {
    background: linear-gradient(135deg, var(--green-dark), #062c22);
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    font-family: 'Segoe UI', sans-serif;
}

.calculator {
    background: #0b2f23;
    padding: 20px;
    border-radius: 20px;
    width: 100%;
    max-width: 380px;
    box-shadow: 0 20px 40px rgba(0,0,0,0.5);
}

#display {
    width: 100%;
    height: 70px;
    background: #031f17;
    border: none;
    border-radius: 12px;
    color: var(--text-light);
    font-size: 2rem;
    text-align: right;
    padding: 15px;
    margin-bottom: 15px;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;
}

button {
    padding: 18px;
    font-size: 1.1rem;
    border: none;
    border-radius: 12px;
    cursor: pointer;
    background: var(--green-soft);
    color: var(--text-light);
    transition: all 0.2s ease;
}

button:hover {
    background: var(--green-light);
    color: #0b2f23;
}

.operator {
    background: var(--green-dark);
}

.equal {
    background: var(--green-light);
    color: #0b2f23;
    grid-row: span 2;
}

.zero {
    grid-column: span 2;
}

.fn {
    background: #124f3a;
}

/* 📱 Responsive */
@media (max-width: 480px) {
    #display {
        font-size: 1.6rem;
        height: 60px;
    }

    button {
        padding: 15px;
        font-size: 1rem;
    }
}