:root {
    --bg: #0a0e27;
    --panel: #0d1230;
    --text: #e8e8ff;
    --text-dim: #a0a0c0;
    --brand: #7a6bff;
    --brand2: #5fd8ff;
    --border: rgba(255,255,255,0.1);
    --success: #4ade80;
    --warning: #fbbf24;
}

* { box-sizing: border-box; }

body {
    margin: 0;
    padding: 0;
    background: var(--bg);
    color: var(--text);
    font-family: Inter, system-ui, -apple-system, sans-serif;
    min-height: 100dvh;
}

.navbar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 12px 24px;
    background: var(--panel);
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    z-index: 100;
}

.navbar-left { display: flex; align-items: center; gap: 16px; }
.navbar-logo { font-weight: 700; font-size: 22px; color: var(--text); }
.navbar-brand { color: var(--text-dim); font-size: 13px; }

.navbar-right { display: flex; gap: 12px; }

.btn {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    padding: 8px 16px;
    border-radius: 8px;
    border: 1px solid var(--border);
    background: rgba(255,255,255,0.04);
    color: var(--text);
    font-size: 14px;
    cursor: pointer;
    transition: all 0.2s;
}

.btn:hover { background: rgba(255,255,255,0.08); }
.btn-primary {
    background: linear-gradient(135deg, var(--brand), #9a6bff);
    border: none;
    color: white;
}

.main-layout {
    display: flex;
    min-height: calc(100dvh - 53px);
}

.sidebar {
    width: 280px;
    background: var(--panel);
    border-right: 1px solid var(--border);
    padding: 24px;
    flex-shrink: 0;
}

.sidebar-header h3 { margin: 0 0 4px; font-size: 18px; }
.sidebar-header .subtitle { color: var(--text-dim); font-size: 13px; margin: 0 0 20px; }

.topic-list { display: flex; flex-direction: column; gap: 4px; }
.topic-item {
    display: flex;
    align-items: center;
    gap: 12px;
    padding: 12px 16px;
    border-radius: 10px;
    cursor: pointer;
    transition: all 0.2s;
    color: var(--text-dim);
}

.topic-item:hover { background: rgba(255,255,255,0.04); color: var(--text); }
.topic-item.active {
    background: rgba(122, 107, 255, 0.15);
    color: var(--text);
    border: 1px solid var(--brand);
}

.topic-item i { width: 20px; }

.sidebar-footer { margin-top: 40px; padding-top: 20px; border-top: 1px solid var(--border); }
.philosophy { font-size: 13px; color: var(--text-dim); line-height: 1.5; }
.philosophy strong { color: var(--text); display: block; margin-bottom: 8px; }

.content {
    flex: 1;
    padding: 32px 40px;
    max-width: 1100px;
    margin: 0 auto;
    width: 100%;
}

.header h1 { font-size: 32px; margin: 0 0 4px; }
.header .tagline { color: var(--text-dim); margin: 0; }

.section {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: 16px;
    padding: 24px;
    margin-bottom: 24px;
}

.section-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: 16px;
}

.section-header h2 {
    margin: 0;
    font-size: 18px;
    display: flex;
    align-items: center;
    gap: 10px;
}

.input-row {
    display: flex;
    gap: 12px;
}

.controls-row {
    display: flex;
    align-items: center;
    gap: 10px;
    margin-bottom: 12px;
    color: var(--text-dim);
    font-size: 14px;
}

.controls-row label {
    font-weight: 700;
    color: var(--text);
}

.controls-row select {
    background: #070a1e;
    border: 1px solid var(--border);
    color: var(--text);
    padding: 10px 12px;
    border-radius: 10px;
    font-size: 14px;
}

.input-row input {
    flex: 1;
    background: #070a1e;
    border: 1px solid var(--border);
    color: var(--text);
    padding: 12px 16px;
    border-radius: 10px;
    font-size: 15px;
}

.output-box {
    background: #070a1e;
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 20px;
    margin-top: 16px;
    line-height: 1.65;
    font-size: 15px;
}

.formatted-output h3 {
    color: var(--brand2);
    font-size: 18px;
    margin: 18px 0 8px;
}

.formatted-output h3:first-child { margin-top: 0; }
.formatted-output p { margin: 0 0 12px; }
.formatted-output ul, .formatted-output ol { margin: 0 0 14px 22px; padding: 0; }
.formatted-output li { margin: 5px 0; }
.formatted-output code, .math-inline {
    background: rgba(95,216,255,0.10);
    border: 1px solid rgba(95,216,255,0.18);
    padding: 1px 5px;
    border-radius: 6px;
    font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
}

.math-block {
    display: block;
    margin: 12px 0;
    padding: 12px 14px;
    border-radius: 10px;
    background: rgba(122,107,255,0.12);
    border: 1px solid rgba(122,107,255,0.22);
    font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
    overflow-x: auto;
}

.problems-grid {
    display: grid;
    gap: 16px;
}

.problem-card {
    background: #070a1e;
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 20px;
}

.problem-card .problem-text {
    font-size: 15px;
    line-height: 1.6;
    margin-bottom: 16px;
}

.problem-card .solution {
    background: rgba(74, 222, 128, 0.08);
    border-left: 3px solid var(--success);
    padding: 12px 16px;
    border-radius: 0 8px 8px 0;
    margin-top: 12px;
    display: none;
}

.problem-card.show-solution .solution { display: block; }

.solver-area textarea {
    width: 100%;
    min-height: 120px;
    background: #070a1e;
    border: 1px solid var(--border);
    color: var(--text);
    padding: 16px;
    border-radius: 12px;
    font-size: 15px;
    resize: vertical;
    margin-bottom: 12px;
}

.solver-actions {
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
}

.hidden { display: none !important; }

@media (max-width: 900px) {
    .main-layout { flex-direction: column; }
    .sidebar { width: 100%; border-right: none; border-bottom: 1px solid var(--border); }
    .content { padding: 24px 20px; }
}