body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    font-family: 'Arial', sans-serif;
    background: #667eea;
    animation: systematicColorChange 20s infinite linear;
}

@keyframes systematicColorChange {
    0% { background: #667eea; } /* Soft Blue */
    25% { background: #764ba2; } /* Deep Purple */
    50% { background: #2c3e50; } /* Dark Slate */
    75% { background: #3498db; } /* Bright Blue */
    100% { background: #667eea; } /* Back to Soft Blue */
}

.calculator {
    background: #1a1a1a;
    border-radius: 20px;
    padding: 20px;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.4);
    width: 360px;
    animation: fadeIn 0.5s ease-in;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(-20px); }
    to { opacity: 1; transform: translateY(0); }
}

.display {
    background: #2c2c2c;
    color: #fff;
    font-size: 2.5em;
    padding: 20px;
    border-radius: 10px;
    text-align: right;
    margin-bottom: 20px;
    overflow-x: auto;
    white-space: nowrap;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

button {
    background: #333;
    color: #fff;
    border: none;
    padding: 20px;
    font-size: 1.5em;
    border-radius: 10px;
    cursor: pointer;
    position: relative;
    transition: transform 0.2s, background 0.2s;
    transform-style: preserve-3d;
}

button:hover {
    background: #555;
    transform: scale(1.1);
    box-shadow: 0 5px 15px rgba(0, 0, 0, 0.3);
}

button:active {
    animation: flip 0.4s ease-in-out;
}

@keyframes flip {
    0% { transform: perspective(400px) rotateY(0deg); }
    50% { transform: perspective(400px) rotateY(180deg); }
    100% { transform: perspective(400px) rotateY(360deg); }
}

.operator {
    background: #ff9500;
}

.operator:hover {
    background: #ffaa33;
}

.equals {
    background: #2196F3;
}

.equals:hover {
    background: #42a5f5;
}

.clear {
    background: #f44336;
}

.clear:hover {
    background: #ef5350;
}

.currency-section {
    margin-top: 20px;
    color: #fff;
}

select {
    width: 100%;
    padding: 10px;
    font-size: 1em;
    border-radius: 5px;
    background: #333;
    color: #fff;
    border: none;
    cursor: pointer;
    transition: background 0.2s;
}

select:hover {
    background: #555;
}

.currency-result {
    margin-top: 10px;
    font-size: 1.2em;
}