body, h1, h2, h3, p, ul, li {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: Arial, sans-serif;
    line-height: 1.6;
    color: #333;
    background-color: #f4f4f4;
    padding-bottom: 60px;
}

header {
    background-color: #333;
    color: #fff;
    padding: 1.5rem 0;
    text-align: center;
}

header h1 {
    margin: 0;
    font-size: 2.5rem;
}

nav {
    background-color: #444;
    padding: 0.5rem 0;
}

nav ul {
    list-style-type: none;
    text-align: center;
}

nav ul li {
    display: inline-block;
    margin-right: 20px;
}

nav ul li:last-child {
    margin-right: 0;
}

nav ul li a {
    color: #fff;
    text-decoration: none;
    padding: 0.5rem 1rem;
    display: block;
}

nav ul li a:hover, nav ul li a.active {
    background-color: #555;
    border-radius: 4px;
}

main {
    padding: 20px;
    max-width: 960px;
    margin: 20px auto;
    background-color: #fff;
    border-radius: 8px;
    box-shadow: 0 0 10px rgba(0,0,0,0.1);
}

section {
    margin-bottom: 30px;
    padding-bottom: 20px;
    border-bottom: 1px solid #eee;
}

section:last-child {
    border-bottom: none;
}

section h2 {
    color: #333;
    margin-bottom: 15px;
    font-size: 2rem;
}

.topic-page h2 {
    color: #0056b3;
}

.topic-introduction, .topic-real-life {
    margin-bottom: 20px;
    font-style: italic;
}

.sub-topic {
    margin-bottom: 25px;
    padding: 15px;
    background-color: #f9f9f9;
    border-left: 4px solid #007bff;
    border-radius: 4px;
    padding-left: 30px;
}

.sub-topic h3 {
    color: #007bff;
    margin-bottom: 10px;
    font-size: 1.5rem;
}

.sub-topic p {
    margin-bottom: 10px;
}

@media (max-width: 768px) {
    nav ul li {
        display: block;
        margin-right: 0;
        margin-bottom: 5px;
    }

    nav ul li a {
        padding: 0.8rem;
    }

    header h1 {
        font-size: 2rem;
    }

    main {
        padding: 15px;
        margin: 15px;
    }

    .sub-topic h3 {
        font-size: 1.3rem;
    }
}