body {
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background: linear-gradient(120deg, hsl(0, 100%, 90%), hsl(240, 100%, 90%), hsl(120, 100%, 90%));
    background-size: 400% 400%;
    animation: gradientAnimation 15s ease infinite;
    font-family: Arial, sans-serif;
}

@keyframes gradientAnimation {
    0% { background-position: 0% 50%; }
    50% { background-position: 100% 50%; }
    100% { background-position: 0% 50%; }
}

#calculator {
    background-color: hsl(0, 0%, 20%);
    border-radius: 20px;
    max-width: 400px;
    width: 100%;
    overflow: hidden;
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.1);
}

#display {
    width: 100%;
    padding: 20px;
    font-size: 3.5rem;
    text-align: right;
    border: none;
    background-color: hsl(0, 0%, 25%);
    color: white;
    box-sizing: border-box;
}

#keys {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 15px;
    padding: 20px;
}

button {
    width: 100%;
    aspect-ratio: 1;
    border-radius: 15px;
    border: none;
    background-color: hsl(0, 0%, 35%);
    color: white;
    font-size: 2rem;
    cursor: pointer;
    transition: background-color 0.3s, transform 0.1s;
    box-shadow: 0 5px 15px rgba(0, 0, 0, 0.1);
}

button:hover {
    background-color: hsl(0, 0%, 45%);
}

button:active {
    background-color: hsl(0, 0%, 50%);
    transform: scale(0.95);
}

.operator-btn {
    background-color: hsl(200, 69%, 65%);
    color: white;
}

.operator-btn:hover {
    background-color: hsl(200, 69%, 55%);
}