CSS:

```
* {
    box-sizing: border-box;
}

body {
    font-family: Arial, sans-serif;
    margin: 0;
}

.container {
    display: flex;
    flex-wrap: wrap;
}

.sidebar {
    background-color: #333;
    color: #fff;
    flex: 0 0 200px;
    height: 100vh;
    padding: 20px;
}

.sidebar ul {
    list-style: none;
    margin: 0;
    padding: 0;
}

.sidebar li {
    margin-bottom: 10px;
}

.sidebar a {
    color: #fff;
    text-decoration: none;
}

.main-content {
    flex: 1;
    padding: 20px;
}

section {
    margin-bottom: 40px;
}

h2 {
    font-size: 50px;
    margin-bottom: 20px;
}

img {
    max-width: 100%;
    height: auto;
}

@media screen and (max-width: 768px) {
    .container {
        flex-direction: column;
    }

    .sidebar {
        flex: 0 0 auto;
        height: auto;
        margin-bottom: 20px;
    }

    .main-content {
        flex: 1;
    }
}
```