* {
    box-sizing: border-box;
    font-family: 'Arial', sans-serif;
    text-align: center;
    font-size: 24px;
    margin: 0;
    padding: 0;
}

body {
    background-image: url(Images/background-img.jpg);
    background-position: center;
    background-repeat: no-repeat;
    background-size: cover;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: #f0f0f0;
}

h1 {
    font-size: 56px;
    margin-bottom: 20px;
}

.calc-body {
    background-color: #333;
    padding: 20px;
    border-radius: 15px;
    box-shadow: 0 0 20px rgba(0, 0, 0, 0.1);
    width: 420px;
}

.display {
    width: 100%;
    height: 60px;
    background-color: #fff;
    border-radius: 10px;
    margin-bottom: 20px;
    font-size: 32px;
    padding: 10px 20px;
    box-shadow: inset 0 0 5px rgba(0, 0, 0, 0.1);
    text-align: right;
    line-height: 40px;
}

.line-five {
    margin-top: 10px;
}

.line-five > button {
    width: 165px;
    background-color: #c0392b;
}

.line-five > button:hover {
    background-color: #e74c3c;
}

.line-five > button:active {
    background-color: #a93226;
}

button {
    width: 75px;
    height: 75px;
    border-radius: 15px;
    margin: 5px;
    border: none;
    outline: none;
    font-size: 24px;
    color: #fff;
    background-color: #666;
    transition: background-color 0.3s, transform 0.1s;
}

button:hover {
    cursor: pointer;
    background-color: #777;
}

button:active {
    background-color: #555;
    transform: scale(0.95);
}

button.key-active {
    box-shadow: 0 0 4px #00ffff, 0 0 4px #00ffff inset;
    transform: scale(1.05);
}

button:nth-child(4n) {
    background-color: #f39c12;
}

button:nth-child(4n):hover {
    background-color: #e67e22;
}

button:nth-child(4n):active {
    background-color: #d35400;
}

#equal-btn {
    background-color: #27ae60;
}

#equal-btn:hover {
    background-color: #2ecc71;
}

#equal-btn:active {
    background-color: #1abc9c;
}