*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
    --clr-bg: #0f172a;
    --clr-surface: #1e293b;
    --clr-accent: #a78bfa;
    --clr-accent2: #f472b6;
    --clr-text: #e2e8f0;
    --clr-muted: #94a3b8;
    --clr-ok: #34d399;
    --clr-bad: #f87171;
    --clr-warn: #fbbf24;
    --clr-border-soft: rgba(255,255,255,.06);
    --clr-border-mid: rgba(255,255,255,.1);
    --clr-border-strong: rgba(255,255,255,.12);
    --clr-header-line: rgba(255,255,255,.05);
    --clr-panel: rgba(255,255,255,.03);
    --clr-panel-2: rgba(255,255,255,.04);
    --clr-input-bg: #0b1120;
    --clr-progress-bg: rgba(255,255,255,.08);
    --clr-btn-secondary: rgba(255,255,255,.06);
    --clr-btn-danger-bg: rgba(248,113,113,.12);
    --clr-btn-danger-text: #fecaca;
    --radius: 14px;
    --shadow: 0 8px 32px rgba(0,0,0,.35);
}

html[data-theme="light"] {
    --clr-bg: #f8fafc;
    --clr-surface: #ffffff;
    --clr-accent: #8b5cf6;
    --clr-accent2: #ec4899;
    --clr-text: #0f172a;
    --clr-muted: #64748b;
    --clr-ok: #10b981;
    --clr-bad: #dc2626;
    --clr-warn: #d97706;
    --clr-border-soft: rgba(15,23,42,.08);
    --clr-border-mid: rgba(15,23,42,.12);
    --clr-border-strong: rgba(15,23,42,.16);
    --clr-header-line: rgba(15,23,42,.08);
    --clr-panel: rgba(15,23,42,.03);
    --clr-panel-2: rgba(15,23,42,.04);
    --clr-input-bg: #ffffff;
    --clr-progress-bg: rgba(15,23,42,.08);
    --clr-btn-secondary: rgba(15,23,42,.04);
    --clr-btn-danger-bg: rgba(220,38,38,.08);
    --clr-btn-danger-text: #b91c1c;
    --shadow: 0 8px 32px rgba(15,23,42,.10);
}

body {
    font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
    background: var(--clr-bg);
    color: var(--clr-text);
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    transition: background-color .25s ease, color .25s ease;
}

.breadcrumb {
    padding: 1rem 2rem;
    font-size: .85rem;
    color: var(--clr-muted);
}
.breadcrumb a {
    color: var(--clr-accent);
    text-decoration: none;
}
.breadcrumb a:hover { text-decoration: underline; }
.breadcrumb span { margin: 0 .4rem; }

header {
    position: relative;
    text-align: center;
    padding: 3rem 2rem 2rem;
    border-bottom: 1px solid var(--clr-header-line);
}
header .icon {
    font-size: 3.2rem;
    display: block;
    margin-bottom: .75rem;
}
header h1 {
    font-size: clamp(1.8rem, 4vw, 2.6rem);
    font-weight: 800;
    background: linear-gradient(90deg, var(--clr-accent), var(--clr-accent2));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}
header .subtitle {
    margin-top: .5rem;
    color: var(--clr-muted);
    font-size: 1.05rem;
    max-width: 720px;
    margin-inline: auto;
    line-height: 1.6;
}

.theme-toggle {
    position: absolute;
    top: 1rem;
    right: 2rem;
    width: 46px;
    height: 46px;
    border-radius: 999px;
    padding: 0;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    background: var(--clr-surface);
    border: 1px solid var(--clr-border-mid);
    color: var(--clr-text);
    cursor: pointer;
    box-shadow: 0 4px 14px rgba(0,0,0,.12);
    font-size: 1.15rem;
    transition: all .2s ease;
    z-index: 10;
}
.theme-toggle:hover {
    transform: translateY(-1px);
    box-shadow: 0 6px 18px rgba(0,0,0,.18);
}
.theme-toggle .sun { display: none; }
.theme-toggle .moon { display: inline; }
html[data-theme="light"] .theme-toggle .sun { display: inline; }
html[data-theme="light"] .theme-toggle .moon { display: none; }

main {
    flex: 1;
    width: 100%;
    max-width: 960px;
    margin-inline: auto;
    padding: 2.5rem 2rem 3rem;
}

.card {
    background: var(--clr-surface);
    border: 1px solid var(--clr-border-soft);
    border-radius: var(--radius);
    padding: 2rem;
    box-shadow: var(--shadow);
}
.card h2 {
    color: var(--clr-accent);
    margin-bottom: 1rem;
    font-size: 1.25rem;
}
.desc {
    color: var(--clr-muted);
    line-height: 1.7;
    margin-bottom: 1rem;
}

.difficulty-bar {
    display: flex;
    flex-wrap: wrap;
    gap: .75rem;
    margin: 1rem 0 1.5rem;
}
.diff-btn {
    border: 1px solid var(--clr-border-mid);
    background: var(--clr-panel-2);
    color: var(--clr-text);
    border-radius: 999px;
    padding: .65rem 1rem;
    cursor: pointer;
    font-weight: 700;
    transition: .2s ease;
}
.diff-btn:hover {
    transform: translateY(-2px);
    border-color: rgba(167,139,250,.4);
}
.diff-btn.active {
    background: linear-gradient(135deg, var(--clr-accent), var(--clr-accent2));
    color: #0f172a;
    border-color: transparent;
}

.task-box {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    gap: 1rem;
    margin: 1.5rem 0 1rem;
    padding: 1.25rem;
    border-radius: 12px;
    background: var(--clr-panel);
    border: 1px solid var(--clr-border-soft);
}
.task-label {
    color: var(--clr-muted);
    font-size: 1rem;
}
.task-number {
    font-family: 'Courier New', monospace;
    font-size: clamp(1.8rem, 5vw, 2.6rem);
    font-weight: 700;
    color: var(--clr-text);
    background: rgba(167,139,250,.1);
    border: 1px solid rgba(167,139,250,.25);
    padding: .5rem .9rem;
    border-radius: 12px;
}
.task-arrow {
    font-size: 1.4rem;
    color: var(--clr-muted);
}
.answer-wrap {
    display: flex;
    flex-wrap: wrap;
    gap: .75rem;
    align-items: center;
}
input[type="text"] {
    background: var(--clr-input-bg);
    color: var(--clr-text);
    border: 1px solid var(--clr-border-strong);
    border-radius: 12px;
    padding: .9rem 1rem;
    font-size: 1.2rem;
    min-width: 250px;
    outline: none;
    font-family: 'Courier New', monospace;
}
input[type="text"]:focus {
    border-color: var(--clr-accent);
    box-shadow: 0 0 0 3px rgba(167,139,250,.15);
}

.btn {
    border: none;
    border-radius: 999px;
    padding: .9rem 1.4rem;
    font-weight: 700;
    cursor: pointer;
    transition: transform .2s;
}
.btn:hover { transform: translateY(-2px); }
.btn-primary {
    background: linear-gradient(135deg, var(--clr-accent), var(--clr-accent2));
    color: #0f172a;
}
.btn-secondary {
    background: var(--clr-btn-secondary);
    color: var(--clr-text);
    border: 1px solid var(--clr-border-soft);
}
.btn-danger {
    background: var(--clr-btn-danger-bg);
    color: var(--clr-btn-danger-text);
    border: 1px solid rgba(248,113,113,.25);
}

.feedback {
    margin-top: 1rem;
    min-height: 2.2rem;
    font-size: 1.08rem;
    font-weight: 600;
    line-height: 1.6;
}
.feedback.ok { color: var(--clr-ok); }
.feedback.bad { color: var(--clr-bad); }
.feedback.hint { color: var(--clr-warn); }

.stats {
    margin-top: 1.25rem;
    padding-top: 1rem;
    border-top: 1px solid var(--clr-border-mid);
    font-size: 1.18rem;
    line-height: 1.9;
}
.stats .ok { color: var(--clr-ok); font-weight: 800; }
.stats .bad { color: var(--clr-bad); font-weight: 800; }
.stats .all { color: var(--clr-text); font-weight: 800; }

.meta {
    display: flex;
    flex-wrap: wrap;
    gap: 1rem 2rem;
    margin-top: 1rem;
    color: var(--clr-muted);
    font-size: .95rem;
}
.help {
    margin-top: 1rem;
    color: var(--clr-muted);
    font-size: .95rem;
    line-height: 1.6;
}

.page-nav {
    display: flex;
    justify-content: space-between;
    gap: 1rem;
    flex-wrap: wrap;
    margin-top: 2rem;
}
.nav-link {
    display: inline-flex;
    align-items: center;
    gap: .5rem;
    padding: .85rem 1.3rem;
    border-radius: 999px;
    text-decoration: none;
    font-weight: 700;
}
.nav-link.outline {
    color: var(--clr-accent);
    border: 1px solid rgba(167,139,250,.4);
}
.nav-link.filled {
    background: linear-gradient(135deg, var(--clr-accent), var(--clr-accent2));
    color: #0f172a;
}

footer {
    text-align: center;
    padding: 1.5rem;
    color: var(--clr-muted);
    font-size: .8rem;
    border-top: 1px solid var(--clr-header-line);
}

.progress { margin-top: 1rem; }
.progress-bar {
    width: 100%;
    height: 14px;
    background: var(--clr-progress-bg);
    border-radius: 999px;
    overflow: hidden;
}
.progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--clr-ok), var(--clr-accent));
    border-radius: 999px;
    transition: width .3s ease;
}

@media (max-width: 700px) {
    header {
        padding-top: 5rem;
    }
    .theme-toggle {
        right: 1rem;
    }
}