body {
    font-family: 'Segoe UI', Arial, sans-serif;
    background: #f6f8fa;
    margin: 0;
    color: #222;
}

header {
    background: #24292e;
    color: #fff;
    padding: 20px 0;
    text-align: center;
}

nav button {
    margin: 0 10px;
    padding: 10px 18px;
    border: none;
    border-radius: 3px;
    background: #0366d6;
    color: white;
    font-size: 1em;
    cursor: pointer;
    transition: background 0.2s;
}
nav button:hover {
    background: #005cc5;
}

main {
    max-width: 900px;
    margin: 40px auto 20px auto;
    padding: 20px;
    background: #fff;
    border-radius: 5px;
    box-shadow: 0 2px 8px rgba(36,41,46,0.07);
}

.course-section {
    display: block;
}

.course-section:not(:first-of-type) {
    margin-top: 40px;
}

h2, h3 {
    color: #0366d6;
}

pre {
    background: #f3f3f3;
    border-radius: 5px;
    padding: 15px;
    overflow-x: auto;
    font-size: 1em;
}

ul, ol {
    margin-left: 30px;
}

footer {
    text-align: center;
    padding: 30px 0 10px 0;
    background: #24292e;
    color: #fff;
    margin-top: 50px;
    font-size: 0.95em;
}

footer a {
    color: #ffe066;
    text-decoration: underline;
}

@media (max-width: 600px) {
    main {
        padding: 10px;
    }
    pre {
        font-size: 0.9em;
    }
}