:root {
    /* Light theme (default) */
    --color-bg: #e5ebf4;
    --color-surface: #ffffff;
    --color-surface-2: #f7f9fc;
    --color-text: #16233b;
    --color-muted: #5c6b84;
    --color-primary: #1b4fd8;
    --color-primary-hover: #1740b4;
    --color-primary-soft: #e7eeff;
    --color-on-primary: #ffffff;
    --color-success: #178a50;
    --color-success-soft: #e3f6ec;
    --color-danger: #d23c3c;
    --color-danger-soft: #fdeaea;
    --color-border: #c4cfe0;
    --color-border-strong: #a9b7cd;

    /* Accent de marque, aligné sur l'application. */
    --color-accent: #c2410c;
    --color-accent-hover: #a5350a;
    --color-on-accent: #ffffff;
    --color-grid-line: #c3cddb;
    --color-grid-strong: #274067;
    --color-cell-given-bg: #eef2f8;
    --color-cell-selected: #cfe0ff;
    --color-cell-peer: #edf3fd;
    --color-cell-error-bg: #fde7e7;

    --shadow-1: 0 1px 2px rgba(16, 30, 54, 0.10), 0 2px 10px rgba(16, 30, 54, 0.09);
    --shadow-2: 0 10px 30px rgba(16, 30, 54, 0.18);

    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 20px;

    font-family: system-ui, -apple-system, "Segoe UI", sans-serif;
    color: var(--color-text);
}

@media (prefers-color-scheme: dark) {
    :root {
        --color-bg: #080d16;
        --color-surface: #16202f;
        --color-surface-2: #1c2839;
        --color-text: #e8edf5;
        --color-muted: #93a1b8;
        --color-primary: #5b8cff;
        --color-primary-hover: #7aa2ff;
        --color-primary-soft: #22304d;
        --color-success: #4cc38a;
        --color-success-soft: #173225;
        --color-danger: #ff7b7b;
        --color-danger-soft: #3a1d24;
        --color-border: #35455e;
        --color-border-strong: #4a5d7a;
        --color-accent: #f97316;
        --color-accent-hover: #fb8c3c;
        --color-on-accent: #1b1005;
        --shadow-1: 0 1px 2px rgba(0, 0, 0, 0.4), 0 2px 10px rgba(0, 0, 0, 0.34);
        --shadow-2: 0 10px 30px rgba(0, 0, 0, 0.5);
        --color-grid-line: #33445d;
        --color-grid-strong: #b9c8e0;
        --color-cell-given-bg: #1d2a3d;
    }
}

html {
    scroll-behavior: smooth;
}

body {
    margin: 0;
    background: var(--color-bg);
    color: var(--color-text);
    transition: background-color 0.2s ease, color 0.2s ease;
}

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

a:hover {
    color: var(--color-primary-hover);
    text-decoration: underline;
}

/* Header & Navigation */
.header {
    position: sticky;
    top: 0;
    background: var(--color-surface);
    border-bottom: 1px solid var(--color-border);
    z-index: 100;
    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.05);
}

.header-content {
    display: flex;
    justify-content: space-between;
    align-items: center;
    max-width: 1200px;
    margin: 0 auto;
    padding: 12px 16px;
    gap: 24px;
}

.logo {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--color-primary);
    text-decoration: none;
    white-space: nowrap;
}

.logo:hover {
    color: var(--color-primary-hover);
}

.nav-links {
    display: flex;
    align-items: center;
    gap: 20px;
    flex: 1;
    list-style: none;
    margin: 0;
    padding: 0;
}

.nav-links a {
    color: var(--color-text);
    font-size: 0.95rem;
    transition: color 0.2s;
}

.nav-links a:hover {
    color: var(--color-primary);
}

.header-cta {
    white-space: nowrap;
}

/* Hero Section */
.hero {
    background: linear-gradient(135deg, var(--color-primary-soft), var(--color-surface-2));
    border-bottom: 4px solid var(--color-accent);
    padding: 80px 16px;
    text-align: center;
}

.hero-content {
    max-width: 600px;
    margin: 0 auto;
}

.hero h1 {
    font-size: clamp(2rem, 5vw, 3.5rem);
    margin: 0 0 16px 0;
    line-height: 1.1;
    letter-spacing: -0.02em;
    font-weight: 800;
}

.hero .subtitle {
    font-size: 1.1rem;
    color: var(--color-muted);
    margin-bottom: 32px;
}

.hero-actions {
    display: flex;
    gap: 12px;
    justify-content: center;
    flex-wrap: wrap;
}

/* Page Layout */
.page {
    max-width: 1200px;
    margin: 0 auto;
    padding: 48px 16px;
}

h1 {
    font-size: clamp(1.5rem, 4vw, 2.5rem);
    margin: 0 0 12px 0;
    line-height: 1.2;
}

h2 {
    font-size: 1.5rem;
    margin: 32px 0 16px 0;
    color: var(--color-text);
}

p {
    line-height: 1.6;
    margin: 0 0 16px 0;
}

.lead {
    font-size: 1.1rem;
    color: var(--color-muted);
    margin-bottom: 24px;
}

.meta {
    color: var(--color-muted);
    font-size: 0.95rem;
    margin-top: 8px;
    margin-bottom: 24px;
}

.message {
    color: var(--color-danger);
    background: var(--color-danger-soft);
    padding: 12px 16px;
    border-radius: var(--radius-sm);
    margin: 16px 0;
}

/* Buttons */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    padding: 12px 24px;
    background: var(--color-accent);
    color: var(--color-on-accent);
    border: none;
    border-radius: var(--radius-sm);
    text-decoration: none;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: background-color 0.2s, transform 0.1s;
    gap: 8px;
    min-height: 46px;
    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.16);
}

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

.btn:active {
    transform: translateY(1px);
}

.btn.secondary {
    background: var(--color-surface-2);
    color: var(--color-text);
    border: 1px solid var(--color-border);
}

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

/* Card Layout */
.card {
    background: var(--color-surface);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-md);
    padding: 24px;
    margin: 16px 0;
}

.card-centered {
    max-width: 600px;
    margin: 32px auto;
}

.difficulty-card {
    background: var(--color-surface);
    border: 1px solid var(--color-border);
    border-top: 4px solid var(--color-accent);
    border-radius: var(--radius-md);
    padding: 24px;
    text-align: center;
    box-shadow: var(--shadow-1);
    transition: transform 0.15s ease, box-shadow 0.2s ease, border-color 0.2s ease;
}

.difficulty-card:hover {
    transform: translateY(-2px);
    border-color: var(--color-border-strong);
    border-top-color: var(--color-accent-hover);
    box-shadow: var(--shadow-2);
}

.difficulty-card h3 {
    font-size: 1.35rem;
    font-weight: 700;
    margin: 0 0 8px 0;
}

.difficulty-card .clues {
    color: var(--color-muted);
    font-size: 0.9rem;
    margin-bottom: 16px;
}

/* Difficulties Grid */
.difficulties {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 20px;
    margin: 32px 0;
    list-style: none;
    padding: 0;
}

.difficulties a {
    display: block;
    padding: 20px;
    background: var(--color-surface);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-md);
    text-align: center;
    text-decoration: none;
    color: var(--color-text);
    transition: all 0.2s;
}

.difficulties a:hover {
    border-color: var(--color-primary);
    box-shadow: 0 4px 12px rgba(47, 107, 255, 0.1);
    transform: translateY(-2px);
}

.difficulties.small {
    display: flex;
    gap: 12px;
    margin: 20px 0;
    align-items: center;
}

.difficulties.small span {
    color: var(--color-muted);
    font-size: 0.9rem;
}

.difficulties.small a {
    padding: 8px 12px;
    font-size: 0.85rem;
}

/* Sudoku Grid */
.sudoku {
    border-collapse: collapse;
    margin: 24px 0;
    background: var(--color-surface);
    border-radius: var(--radius-md);
    overflow: hidden;
    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.08);
    font-variant-numeric: tabular-nums;
}

.sudoku td {
    width: 44px;
    height: 44px;
    text-align: center;
    font-size: 20px;
    border: 1px solid var(--color-grid-line);
    background: var(--color-surface);
    padding: 0;
    font-weight: 500;
}

.sudoku td.given {
    font-weight: 700;
    background: var(--color-cell-given-bg);
}

.sudoku tr:nth-child(3n) td {
    border-bottom: 3px solid var(--color-grid-strong);
}

.sudoku tr:nth-child(3n + 1) td {
    border-top: 3px solid var(--color-grid-strong);
}

.sudoku td:nth-child(3n) {
    border-right: 3px solid var(--color-grid-strong);
}

.sudoku td:nth-child(3n + 1) {
    border-left: 3px solid var(--color-grid-strong);
}

/* Actions */
.actions {
    display: flex;
    gap: 12px;
    margin: 24px 0;
    flex-wrap: wrap;
}

/* Section Grid for "How it works" */
.how-it-works {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 24px;
    margin: 32px 0;
}

.how-it-works-item {
    padding: 20px;
    background: var(--color-surface);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-md);
}

.how-it-works-item h3 {
    margin: 0 0 12px 0;
    font-size: 1.1rem;
}

/* Footer */
.footer {
    background: var(--color-surface-2);
    border-top: 1px solid var(--color-border);
    padding: 48px 16px;
}

.footer-content {
    max-width: 1200px;
    margin: 0 auto;
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 40px;
    margin-bottom: 32px;
}

.footer-section h4 {
    font-size: 0.9rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--color-muted);
    margin: 0 0 16px 0;
}

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

.footer-section li {
    margin-bottom: 8px;
}

.footer-section a {
    color: var(--color-text);
    text-decoration: none;
    font-size: 0.9rem;
}

.footer-section a:hover {
    color: var(--color-primary);
}

.footer-bottom {
    text-align: center;
    padding-top: 32px;
    border-top: 1px solid var(--color-border);
    color: var(--color-muted);
    font-size: 0.85rem;
}

/* Responsive */
@media (max-width: 768px) {
    .header-content {
        flex-direction: column;
        gap: 12px;
    }

    .nav-links {
        width: 100%;
        justify-content: space-around;
        gap: 12px;
    }

    .hero {
        padding: 60px 16px;
    }

    .hero h1 {
        font-size: 2rem;
    }

    .hero-actions {
        flex-direction: column;
    }

    .hero .btn {
        width: 100%;
    }

    .page {
        padding: 32px 16px;
    }

    .difficulties {
        grid-template-columns: 1fr;
    }

    .footer-content {
        grid-template-columns: 1fr;
        gap: 24px;
    }
}

@media (max-width: 480px) {
    .header-content {
        padding: 8px 12px;
    }

    .logo {
        font-size: 1rem;
    }

    .nav-links {
        font-size: 0.8rem;
        gap: 8px;
    }

    .hero {
        padding: 48px 12px;
    }

    .hero h1 {
        font-size: 1.5rem;
    }

    .page {
        padding: 20px 12px;
    }

    h1 {
        font-size: 1.25rem;
    }

    h2 {
        font-size: 1.1rem;
    }

    .sudoku {
        width: min(92vw, 420px);
        margin: 16px auto;
    }

    .sudoku td {
        width: clamp(32px, 8vw, 44px);
        height: clamp(32px, 8vw, 44px);
        font-size: 16px;
    }

    .actions {
        flex-direction: column;
        gap: 8px;
    }

    .actions .btn {
        width: 100%;
    }

    .difficulties.small {
        flex-direction: column;
        align-items: stretch;
    }

    .difficulties.small a {
        text-align: center;
    }
}

@media print {
    .header,
    .hero,
    .actions,
    .difficulties,
    .footer,
    .how-it-works {
        display: none;
    }

    body {
        background: white;
    }

    .page {
        padding: 0;
        max-width: 100%;
    }

    .sudoku {
        margin: 0;
        box-shadow: none;
    }
}