:root {
    --bg-color: #0d0f12;
    --sidebar-bg: #14171c;
    --card-bg: rgba(255, 255, 255, 0.03);
    --card-border: rgba(255, 255, 255, 0.08);
    --text-main: #e2e8f0;
    --text-muted: #94a3b8;
    --primary: #f97316; /* Orange accent */
    --primary-hover: #ea580c;
    --primary-glow: rgba(249, 115, 22, 0.2);
}

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

body {
    font-family: 'Inter', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-main);
    line-height: 1.6;
    height: 100vh;
    overflow: hidden;
}

.app-layout {
    display: flex;
    height: 100vh;
}

/* Sidebar */
.sidebar {
    width: 260px;
    background-color: var(--sidebar-bg);
    border-right: 1px solid var(--card-border);
    display: flex;
    flex-direction: column;
    overflow-y: auto;
}

.logo {
    padding: 24px 24px 8px;
    font-size: 24px;
    font-weight: 700;
    letter-spacing: -0.5px;
}

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

.subtitle {
    padding: 0 24px 24px;
    font-size: 13px;
    color: var(--text-muted);
    font-weight: 500;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    border-bottom: 1px solid var(--card-border);
}

.contest-tabs {
    padding: 16px 24px 0;
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.tab-btn {
    background: rgba(255, 255, 255, 0.05);
    border: 1px solid var(--card-border);
    color: var(--text-muted);
    padding: 8px;
    border-radius: 6px;
    cursor: pointer;
    font-size: 13px;
    font-weight: 600;
    transition: all 0.2s;
    width: 100%;
}

.tab-row {
    display: flex;
    gap: 8px;
}

.tab-btn.sub {
    flex: 1;
}

.tab-btn:hover {
    background: rgba(255, 255, 255, 0.1);
}

.tab-btn.active {
    background: rgba(249, 115, 22, 0.1);
    color: var(--primary);
    border-color: rgba(249, 115, 22, 0.3);
}

.nav-section {
    padding: 24px 0;
}

.nav-section h3 {
    padding: 0 24px 12px;
    font-size: 12px;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 1px;
}

.year-list {
    list-style: none;
}

.year-list li {
    padding: 10px 24px;
    cursor: pointer;
    font-size: 15px;
    font-weight: 500;
    color: var(--text-muted);
    transition: all 0.2s;
    border-left: 3px solid transparent;
}

.year-list li:hover {
    background-color: rgba(255, 255, 255, 0.03);
    color: var(--text-main);
}

.year-list li.active {
    color: var(--primary);
    background-color: var(--primary-glow);
    border-left-color: var(--primary);
}

/* Main Content Area */
.content-area {
    flex: 1;
    overflow-y: auto;
    position: relative;
    padding: 40px;
    scroll-behavior: smooth;
}

.content-header {
    margin-bottom: 40px;
    position: relative;
}

.content-header h2 {
    font-size: 32px;
    font-weight: 700;
    letter-spacing: -1px;
}

.header-glow {
    position: absolute;
    top: -50px;
    left: -50px;
    width: 200px;
    height: 200px;
    background: radial-gradient(circle, var(--primary-glow) 0%, rgba(249,115,22,0) 70%);
    pointer-events: none;
    z-index: -1;
}

.problems-list {
    display: flex;
    flex-direction: column;
    gap: 24px;
    max-width: 800px;
}

.problem-card {
    background-color: #ffffff;
    color: #1a1a1a;
    border: 1px solid var(--card-border);
    border-radius: 12px;
    padding: 32px;
    transition: transform 0.2s, box-shadow 0.2s;
}

.problem-card:hover {
    border-color: rgba(255, 255, 255, 0.15);
    box-shadow: 0 8px 32px rgba(0, 0, 0, 0.4);
}

.problem-header {
    margin-bottom: 20px;
}

.badge {
    background: rgba(249, 115, 22, 0.1);
    color: var(--primary);
    padding: 0.25rem 0.75rem;
    border-radius: 99px;
    font-size: 0.875rem;
    font-weight: 600;
    border: 1px solid rgba(249, 115, 22, 0.2);
}

.tag-container {
    display: flex;
    flex-wrap: wrap;
    gap: 0.4rem;
    justify-content: flex-end;
}

.tag-chip {
    padding: 0.2rem 0.6rem;
    border-radius: 12px;
    font-size: 0.75rem;
    font-weight: 600;
    white-space: nowrap;
}

.tag-chip.primary {
    background: rgba(37, 99, 235, 0.1);
    color: #2563eb;
    border: 1px solid rgba(37, 99, 235, 0.2);
}

.tag-chip.sub {
    background: rgba(16, 185, 129, 0.1);
    color: #059669;
    border: 1px solid rgba(16, 185, 129, 0.2);
}

.tag-chip.meta {
    background: rgba(139, 92, 246, 0.1);
    color: #7c3aed;
    border: 1px solid rgba(139, 92, 246, 0.2);
}

.problem-text {
    font-size: 16px;
    line-height: 1.8;
    margin-bottom: 20px;
    overflow-x: auto;
}

.problem-image {
    max-width: 100%;
    margin-top: 1rem;
    border-radius: 8px;
    background: white;
}

.similar-accordion {
    margin-top: 1.5rem;
    border-top: 1px solid rgba(255,255,255,0.05);
    padding-top: 1rem;
}

.similar-btn {
    background: transparent;
    border: none;
    color: var(--text-secondary);
    font-size: 0.85rem;
    font-weight: 600;
    cursor: pointer;
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

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

.similar-content {
    margin-top: 1rem;
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.similar-problem-card {
    background: var(--surface-light);
    border: 1px solid rgba(255,255,255,0.05);
    padding: 1rem;
    border-radius: 8px;
}

.similar-problem-header {
    display: flex;
    justify-content: space-between;
    font-size: 0.8rem;
    color: var(--text-secondary);
    margin-bottom: 0.5rem;
}

.similar-problem-text {
    font-size: 0.9rem;
    color: var(--text);
    overflow-x: auto;
}

.problem-choices {
    margin-top: 16px;
    padding: 16px;
    background-color: #f8fafc;
    border: 1px solid #e2e8f0;
    border-radius: 8px;
    font-size: 15px;
    color: #334155;
    overflow-x: auto;
}

.problem-actions {
    margin-top: 24px;
    padding-top: 24px;
    border-top: 1px solid #e2e8f0;
}

.btn-secondary {
    background: transparent;
    color: #1a1a1a;
    border: 1px solid #cbd5e1;
    padding: 8px 16px;
    border-radius: 6px;
    font-size: 14px;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s;
}

.btn-secondary:hover {
    background: #f1f5f9;
    border-color: #94a3b8;
}

.problem-solution {
    margin-top: 24px;
    overflow: hidden;
    transition: max-height 0.3s ease-out;
}

.problem-solution.hidden {
    display: none;
}

.solution-content {
    background-color: rgba(249, 115, 22, 0.05);
    border-left: 3px solid var(--primary);
    padding: 20px;
    border-radius: 0 8px 8px 0;
    font-size: 15px;
    color: #334155;
    overflow-x: auto;
}

.empty-state, .loading, .error {
    text-align: center;
    padding: 40px;
    color: var(--text-muted);
    font-weight: 500;
}