:root {
    /* Light Theme Variables */
    --bg-color: #ffffff;
    --text-color: #333333;
    --heading-color: #1a1a1a;
    --primary-color: #0066cc;
    --secondary-color: #f0f4f8;
    --accent-color: #0056b3;
    --border-color: #e1e4e8;
    --code-bg: #f6f8fa;
    --quote-border: #0066cc;
    --table-stripe: #f8f9fa;
    --math-bg: #fff;
    --shadow-color: rgba(0, 0, 0, 0.1);
    --card-bg: #ffffff;
}

[data-theme="dark"] {
    /* Dark Theme Variables */
    --bg-color: #181818;
    --text-color: #c9d1d9;
    --heading-color: #ffffff;
    --primary-color: #58a6ff;
    --secondary-color: #161b22;
    --accent-color: #1f6feb;
    --border-color: #30363d;
    --code-bg: #161b22;
    --quote-border: #1f6feb;
    --table-stripe: #161b22;
    --math-bg: #0d1117;
    --shadow-color: rgba(0, 0, 0, 0.5);
    --card-bg: #161b22;
}

* {
    box-sizing: border-box;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    line-height: 1.7;
    color: var(--text-color);
    background-color: var(--bg-color);
    margin: 0;
    transition: background-color 0.3s, color 0.3s;
}

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

h1, h2, h3, h4, h5, h6 {
    color: var(--heading-color);
    margin-top: 2em;
    margin-bottom: 0.8em;
    font-weight: 700;
    line-height: 1.3;
}

h1 {
    font-size: 2.5em;
    border-bottom: 2px solid var(--border-color);
    padding-bottom: 0.3em;
    margin-top: 1em;
}

h2 {
    font-size: 1.8em;
    border-bottom: 1px solid var(--border-color);
    padding-bottom: 0.3em;
}

h3 {
    font-size: 1.4em;
}

p {
    margin-bottom: 1.2em;
}

a {
    color: var(--primary-color);
    text-decoration: none;
}

a:hover {
    text-decoration: underline;
}

blockquote {
    margin: 1.5em 0;
    padding: 1em;
    background-color: var(--secondary-color);
    border-left: 5px solid var(--quote-border);
    border-radius: 4px;
    font-style: italic;
    color: var(--text-color);
}

code {
    font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace;
    background-color: var(--code-bg);
    padding: 0.2em 0.4em;
    border-radius: 4px;
    font-size: 0.9em;
}

pre {
    background-color: var(--code-bg);
    padding: 1rem;
    border-radius: 6px;
    overflow-x: auto;
    border: 1px solid var(--border-color);
}

pre code {
    background-color: transparent;
    padding: 0;
}

hr {
    border: 0;
    height: 1px;
    background: var(--border-color);
    margin: 3rem 0;
}

/* Tables */
table {
    width: 100%;
    margin-bottom: 1.5rem;
    border-collapse: collapse;
    background-color: var(--card-bg);
    border-radius: 6px;
    overflow: hidden;
    box-shadow: 0 1px 3px var(--shadow-color);
}

th, td {
    padding: 12px 15px;
    text-align: left;
    border-bottom: 1px solid var(--border-color);
}

th {
    background-color: var(--secondary-color);
    font-weight: 600;
    color: var(--heading-color);
}

tr:last-child td {
    border-bottom: none;
}

tr:nth-child(even) {
    background-color: var(--table-stripe);
}

/* Images */
img {
    max-width: 100%;
    height: auto;
    display: block;
    margin: 2rem auto;
    border-radius: 8px;
    box-shadow: 0 4px 6px var(--shadow-color);
    background-color: white; /* Ensure transparent pngs look okay in dark mode if needed */
}

/* Problems gallery */
.page-header {
    display: flex;
    align-items: flex-start;
    justify-content: space-between;
    gap: 1rem;
    flex-wrap: wrap;
    margin-top: 0.5rem;
}

.page-header .title {
    margin: 0;
    border-bottom: none;
    padding-bottom: 0;
}

.page-header .subtitle {
    margin: 0.5rem 0 0;
    opacity: 0.85;
}

.page-actions {
    display: flex;
    gap: 0.75rem;
    align-items: center;
}

.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
    padding: 0.55rem 0.85rem;
    border-radius: 10px;
    border: 1px solid var(--border-color);
    background: var(--card-bg);
    color: var(--text-color);
    text-decoration: none;
    box-shadow: 0 2px 5px var(--shadow-color);
    transition: transform 0.2s ease, background-color 0.2s ease, border-color 0.2s ease;
    white-space: nowrap;
}

.btn:hover {
    transform: translateY(-1px);
    text-decoration: none;
    border-color: var(--primary-color);
    background: var(--secondary-color);
}

.gallery-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
    gap: 16px;
    margin-top: 1.25rem;
}

.gallery-item {
    display: block;
    border-radius: 12px;
    overflow: hidden;
    border: 1px solid var(--border-color);
    background: var(--card-bg);
    box-shadow: 0 6px 14px var(--shadow-color);
    transition: transform 0.2s ease, border-color 0.2s ease, box-shadow 0.2s ease;
}

.gallery-item:hover {
    transform: translateY(-2px);
    border-color: var(--primary-color);
    box-shadow: 0 10px 18px var(--shadow-color);
    text-decoration: none;
}

.gallery-item img {
    margin: 0;
    border-radius: 0;
    box-shadow: none;
    background-color: transparent;
    width: 100%;
    height: 180px;
    object-fit: cover;
}

.gallery-caption {
    padding: 10px 12px;
    font-size: 0.95rem;
    color: var(--text-color);
    opacity: 0.9;
}

.lightbox-overlay {
    position: fixed;
    inset: 0;
    background: rgba(0, 0, 0, 0.75);
    display: none;
    align-items: center;
    justify-content: center;
    padding: 18px;
    z-index: 2000;
}

.lightbox-overlay.open {
    display: flex;
}

.lightbox {
    width: min(1100px, 96vw);
    max-height: 92vh;
    background: var(--bg-color);
    border: 1px solid var(--border-color);
    border-radius: 14px;
    overflow: hidden;
    box-shadow: 0 18px 40px rgba(0,0,0,0.35);
}

.lightbox-toolbar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 12px;
    padding: 10px 12px;
    background: var(--secondary-color);
    border-bottom: 1px solid var(--border-color);
}

.lightbox-title {
    font-weight: 600;
    color: var(--heading-color);
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

.lightbox-actions {
    display: inline-flex;
    gap: 8px;
    align-items: center;
}

.lightbox-btn {
    border: 1px solid var(--border-color);
    background: var(--card-bg);
    color: var(--text-color);
    border-radius: 10px;
    padding: 0.45rem 0.65rem;
    cursor: pointer;
    box-shadow: 0 2px 5px var(--shadow-color);
    transition: transform 0.2s ease, border-color 0.2s ease, background-color 0.2s ease;
}

.lightbox-btn:hover {
    transform: translateY(-1px);
    border-color: var(--primary-color);
    background: var(--secondary-color);
}

.lightbox-body {
    padding: 12px;
    display: grid;
    place-items: center;
}

.lightbox-body img {
    margin: 0;
    border-radius: 10px;
    box-shadow: 0 10px 22px var(--shadow-color);
    background-color: white;
    max-width: 100%;
    max-height: calc(92vh - 64px);
    width: auto;
    height: auto;
}

@media (max-width: 600px) {
    .gallery-item img {
        height: 150px;
    }
}

/* Lists */
ul, ol {
    padding-left: 2em;
    margin-bottom: 1.5em;
}

li {
    margin-bottom: 0.5em;
}

li::marker {
    color: var(--primary-color);
}

/* Math blocks styling */
.math-block {
    overflow-x: auto;
    margin: 1.5em 0;
    padding: 1rem;
    background-color: var(--secondary-color);
    border-radius: 8px;
    text-align: center;
    border: 1px solid var(--border-color);
}

/* UI Elements managed by JS */
.theme-toggle-btn {
    position: fixed;
    top: 20px;
    right: 20px;
    background-color: var(--card-bg);
    border: 1px solid var(--border-color);
    color: var(--text-color);
    padding: 10px;
    border-radius: 50%;
    width: 44px;
    height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
    box-shadow: 0 2px 5px var(--shadow-color);
    cursor: pointer;
    z-index: 1000;
    transition: all 0.2s ease;
    font-size: 1.2rem;
}

.theme-toggle-btn:hover {
    transform: scale(1.1);
    background-color: var(--secondary-color);
}

.toc-container {
    background-color: var(--secondary-color);
    padding: 1.5rem;
    border-radius: 8px;
    margin-bottom: 2rem;
    border: 1px solid var(--border-color);
}

.toc-container h3 {
    margin-top: 0;
    font-size: 1.2rem;
}

.toc-list {
    list-style: none;
    padding-left: 0;
    max-height: 400px;
    overflow-y: auto;
}

.toc-list li {
    margin-bottom: 0.5rem;
}

.toc-list a {
    color: var(--text-color);
    text-decoration: none;
    font-size: 0.95rem;
    display: block;
    padding: 2px 0;
    border-left: 2px solid transparent;
    padding-left: 10px;
}

.toc-list a:hover {
    color: var(--primary-color);
    border-left-color: var(--primary-color);
}

.toc-list .toc-subitem {
    padding-left: 25px;
    font-size: 0.9rem;
    color: var(--text-color);
    opacity: 0.8;
}

.toc-list .toc-h3 {
    padding-left: 40px;
    font-size: 0.85rem;
}

.toc-list .toc-h4, .toc-list .toc-h5, .toc-list .toc-h6 {
    padding-left: 55px;
    font-size: 0.85rem;
}

body[data-page="problems"] .toc-container {
    display: none !important;
}

/* Responsive */
@media (max-width: 600px) {
    .container {
        padding: 1rem;
    }
    
    h1 {
        font-size: 2em;
    }

    .theme-toggle-btn {
        top: auto;
        bottom: 20px;
        right: 20px;
    }
}

/* Print Styles */
@media print {
    .theme-toggle-btn, .back-to-top {
        display: none !important;
    }
    body {
        background-color: white !important;
        color: black !important;
    }
    .math-block, blockquote, pre {
        border: 1px solid #ccc;
        box-shadow: none;
    }
    a {
        text-decoration: none;
        color: black;
    }
}

/* Custom Scrollbar for Webkit */
::-webkit-scrollbar {
    width: 10px;
    height: 10px;
}
::-webkit-scrollbar-track {
    background: var(--bg-color);
}
::-webkit-scrollbar-thumb {
    background: var(--border-color);
    border-radius: 5px;
}
::-webkit-scrollbar-thumb:hover {
    background: var(--text-color);
}

/* Back to Top Button */
.back-to-top {
    position: fixed;
    bottom: 20px;
    right: 80px;
    background-color: var(--primary-color);
    color: white;
    width: 44px;
    height: 44px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.3s, transform 0.2s;
    box-shadow: 0 4px 10px var(--shadow-color);
    border: none;
    z-index: 999;
    font-size: 1.2rem;
}

.back-to-top.visible {
    opacity: 1;
    pointer-events: auto;
}

.back-to-top:hover {
    transform: translateY(-2px);
    background-color: var(--accent-color);
}

/* Mobile Tweaks */
@media (max-width: 600px) {
    .back-to-top {
        right: 75px; 
        bottom: 20px;
        width: 40px;
        height: 40px;
    }
}