* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: 'Arial', sans-serif;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: linear-gradient(135deg, #6e7e85, #2d4f4f);
    color: #f4f4f9;
    padding: 20px;
}

.calculator {
    background-color: #1e1e1e;
    padding: 30px;
    border-radius: 15px;
    width: 100%;
    max-width: 600px;
    text-align: center;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.2);
}

.display input {
    width: 100%;
    padding: 20px;
    border: none;
    background-color: #333333;
    color: #f4f4f9;
    font-size: 2em;
    text-align: right;
    border-radius: 8px;
    margin-bottom: 20px;
    box-shadow: inset 0 4px 10px rgba(0, 0, 0, 0.3);
}

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

button {
    padding: 20px;
    background-color: #444444;
    color: #f4f4f9;
    border: none;
    border-radius: 8px;
    font-size: 1.3em;
    cursor: pointer;
    transition: background-color 0.3s ease, transform 0.1s ease;
    outline: none;
}

button:hover {
    background-color: #616161;
    transform: scale(1.05);
}

button:active {
    background-color: #757575;
    transform: scale(0.98);
}

button:focus {
    box-shadow: 0 0 0 3px #00adb5;
}

.operation, .equals {
    background-color: #00adb5;
    color: #121212;
}

.operation:hover, .equals:hover {
    background-color: #009b96;
}

.equals {
    grid-column: span 1;
    grid-row: span 3;
    font-size: 1.7em;
}

.zero {
    grid-column: span 2;
}

.error {
    background-color: #d32f2f;
    color: white;
    text-align: center;
}

.calculator {
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.4);
    border-radius: 12px;
    overflow: hidden;
}