* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background: linear-gradient(135deg, #0f172a, #1e293b, #334155);
}

.calculator {
    width: 360px;
    background: #1e1e2f;
    border-radius: 20px;
    padding: 25px;
    box-shadow: 0 15px 35px rgba(0,0,0,0.35);
}

#display {
    width: 100%;
    height: 70px;
    border: none;
    outline: none;
    border-radius: 12px;
    background: #f8fafc;
    color: #111827;
    font-size: 2rem;
    text-align: right;
    padding: 15px;
    margin-bottom: 20px;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 15px;
}

button {
    height: 65px;
    border: none;
    border-radius: 12px;
    font-size: 1.3rem;
    cursor: pointer;
    background: #374151;
    color: white;
    transition: 0.25s;
}

button:hover {
    transform: translateY(-3px);
    background: #4b5563;
}

button:active {
    transform: scale(0.95);
}

.operator {
    background: #f59e0b;
}

.operator:hover {
    background: #d97706;
}

.equal {
    grid-column: span 2;
    background: #10b981;
}

.equal:hover {
    background: #059669;
}

@media (max-width: 420px) {
    .calculator {
        width: 95%;
    }

    button {
        height: 55px;
    }

    #display {
        font-size: 1.6rem;
    }
}