:root {
    --bg-color: #ffffff;
    --text-color: #333333;
    --header-bg: #f8f9fa;
    --header-border: #e9ecef;
    --section-bg: #ffffff;
    --section-border: #dee2e6;
    --accent-color: #0056b3;
}

[data-theme="dark"] {
    --bg-color: #121212;
    --text-color: #e0e0e0;
    --header-bg: #1f1f1f;
    --header-border: #333333;
    --section-bg: #1e1e1e;
    --section-border: #333333;
    --accent-color: #4da3ff;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    line-height: 1.6;
    color: var(--text-color);
    background-color: var(--bg-color);
    margin: 0;
    padding: 0;
    transition: background-color 0.3s, color 0.3s;
}

header {
    background-color: var(--header-bg);
    border-bottom: 1px solid var(--header-border);
    padding: 1rem 0;
    position: sticky;
    top: 0;
    z-index: 1000;
}

.header-content {
    max-width: 800px;
    margin: 0 auto;
    padding: 0 1rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
    gap: 1rem;
}

h1 {
    font-size: 1.5rem;
    margin: 0;
}

.theme-toggle select {
    padding: 0.25rem 0.5rem;
    border-radius: 4px;
    border: 1px solid var(--section-border);
    background-color: var(--bg-color);
    color: var(--text-color);
    font-size: 1rem;
}

main {
    max-width: 800px;
    margin: 2rem auto;
    padding: 0 1rem;
}

section {
    background-color: var(--section-bg);
    border: 1px solid var(--section-border);
    border-radius: 8px;
    padding: 1.5rem;
    margin-bottom: 2rem;
    box-shadow: 0 2px 4px rgba(0,0,0,0.05);
}

h2 {
    color: var(--accent-color);
    margin-top: 0;
    border-bottom: 2px solid var(--section-border);
    padding-bottom: 0.5rem;
}

h3 {
    margin-top: 1.5rem;
}

ul, ol {
    margin-bottom: 1.5rem;
}

li {
    margin-bottom: 0.5rem;
}

.responsive-img {
    max-width: 100%;
    height: auto;
    border-radius: 4px;
    border: 1px solid var(--section-border);
    display: block;
    margin: 1.5rem 0 0 0;
}

@media (max-width: 600px) {
    .header-content {
        flex-direction: column;
        align-items: flex-start;
    }
}