:root {
    --primary-green: #2E7D32;
    --secondary-green: #4CAF50;
    --accent-yellow-green: #AED581; /* Lighter yellow-green */
    --text-black: #1a1a1a;
    --bg-white: #ffffff;
    --bg-light: #f1f8e9;
}

body {
    font-family: "Helvetica Neue", Arial, "Hiragino Kaku Gothic ProN", "Hiragino Sans", Meiryo, sans-serif;
    line-height: 1.6;
    color: var(--text-black);
    background-color: var(--bg-light);
    margin: 0;
    padding: 0;
}

header {
    background-color: var(--primary-green);
    color: var(--bg-white);
    padding: 1rem 0;
    text-align: center;
    box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

header h1 {
    margin: 0;
    font-size: 1.8rem;
}

nav {
    background-color: var(--secondary-green);
    padding: 0.5rem 0;
    position: sticky;
    top: 0;
    z-index: 100;
}

nav ul {
    list-style: none;
    margin: 0;
    padding: 0;
    display: flex;
    justify-content: center;
    flex-wrap: wrap;
    gap: 15px;
}

nav ul li {
    margin: 0;
}

nav a {
    text-decoration: none;
    color: var(--bg-white);
    font-weight: bold;
    padding: 0.5rem 1rem;
    border-radius: 4px;
    transition: background-color 0.3s;
}

nav a:hover, nav a.active {
    background-color: var(--accent-yellow-green);
    color: var(--primary-green);
}

.container {
    max-width: 800px;
    margin: 2rem auto;
    padding: 2rem;
    background-color: var(--bg-white);
    border-radius: 8px;
    box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}

h2 {
    color: var(--primary-green);
    border-bottom: 2px solid var(--accent-yellow-green);
    padding-bottom: 0.5rem;
    margin-top: 2rem;
}

h3 {
    color: var(--secondary-green);
    margin-top: 1.5rem;
    border-left: 5px solid var(--accent-yellow-green);
    padding-left: 10px;
}

h4 {
    color: var(--text-black);
    font-weight: bold;
    margin-top: 1.2rem;
}

p {
    margin-bottom: 1rem;
}

ul, ol {
    margin-bottom: 1rem;
    padding-left: 20px;
}

li {
    margin-bottom: 0.5rem;
}

footer {
    text-align: center;
    padding: 2rem 0;
    background-color: var(--text-black);
    color: var(--bg-white);
    margin-top: 3rem;
}

/* Specific button styles for navigation between chapters if needed */
.chapter-nav {
    display: flex;
    justify-content: space-between;
    margin-top: 2rem;
    padding-top: 1rem;
    border-top: 1px solid #eee;
}

.btn {
    display: inline-block;
    padding: 0.5rem 1rem;
    background-color: var(--primary-green);
    color: white;
    text-decoration: none;
    border-radius: 4px;
}

.btn:hover {
    background-color: var(--secondary-green);
}

/* Responsive adjustments */
@media (max-width: 600px) {
    header h1 {
        font-size: 1.4rem;
    }
    
    nav ul {
        flex-direction: column;
        align-items: center;
        gap: 5px;
    }

    nav a {
        display: block;
        width: 100%;
        text-align: center;
    }
}