:root{--bg:#f5f6f8;--surface:#fff;--border:#d9dce1;--text:#1c1f24;--text-muted:#6b7280;--accent:#2f6feb;--accent-contrast:#fff;--error-bg:#fdeceb;--error-border:#e5534b;--error-text:#8a1f16;--sans:system-ui, "Segoe UI", Roboto, sans-serif;--mono:ui-monospace, Consolas, monospace;--lightningcss-light:initial;--lightningcss-dark: ;color-scheme:light;font:16px/1.5 var(--sans)}:root[data-theme=dark]{--bg:#14161a;--surface:#1b1e24;--border:#2e333c;--text:#e6e8eb;--text-muted:#9aa1ac;--accent:#5b9dff;--accent-contrast:#0b1220;--error-bg:#3a1c1c;--error-border:#e5534b;--error-text:#ffb4ac;--lightningcss-light: ;--lightningcss-dark:initial;color-scheme:dark}body{background:var(--bg);color:var(--text);margin:0}.app{box-sizing:border-box;max-width:1100px;margin:0 auto;padding:20px 24px 48px}.site-header{align-items:center;gap:10px;margin-bottom:12px;display:flex}.site-header__brand{color:var(--accent);white-space:nowrap;font-size:15px;font-weight:700;text-decoration:none}.site-header__brand:hover{text-decoration:underline}.site-header__divider{color:var(--text-muted);font-weight:400}.site-header__title{color:var(--text-muted);white-space:nowrap;font-size:15px;font-weight:600}.toolbar-spacer{flex:1}.btn,.toggle-btn{border:1px solid var(--border);background:var(--surface);color:var(--text);cursor:pointer;border-radius:6px;padding:6px 12px;font-size:13px}.btn:hover,.toggle-btn:hover{border-color:var(--accent)}.btn:disabled{opacity:.5;cursor:not-allowed}.icon-btn{padding:6px 10px}.app__intro{color:var(--text-muted);margin:0 0 20px}.app-layout{align-items:flex-start;gap:24px;display:flex}@media (width<=720px){.app-layout{flex-direction:column}}.lesson-sidebar{background:var(--surface);border:1px solid var(--border);border-radius:8px;flex:0 0 220px;overflow:hidden}@media (width<=720px){.lesson-sidebar{box-sizing:border-box;flex:1;width:100%}}.lesson-sidebar ul{margin:0;padding:0;list-style:none}.lesson-sidebar__item{box-sizing:border-box;border:none;border-bottom:1px solid var(--border);width:100%;color:var(--text);text-align:left;cursor:pointer;background:0 0;justify-content:space-between;align-items:center;padding:10px 14px;font-size:13px;display:flex}.lesson-sidebar li:last-child .lesson-sidebar__item{border-bottom:none}.lesson-sidebar__item:hover{background:var(--bg)}.lesson-sidebar__item--active{background:var(--accent);color:var(--accent-contrast)}.lesson-sidebar__checkmark{color:var(--accent);font-weight:700}.lesson-sidebar__item--active .lesson-sidebar__checkmark{color:var(--accent-contrast)}.lesson-view{background:var(--surface);border:1px solid var(--border);box-sizing:border-box;border-radius:8px;flex:1;min-width:0;padding:20px 24px}.lesson-view h2{margin-top:0}.lesson-view__key-points{background:var(--bg);border:1px solid var(--border);border-radius:6px;padding:12px 16px 12px 32px;font-size:14px}.lesson-view__actions{flex-wrap:wrap;justify-content:space-between;align-items:center;gap:12px;margin-top:20px;display:flex}.lesson-view__complete-btn--done{border-color:var(--accent);color:var(--accent)}.lesson-view__nav{gap:8px;display:flex}.playground{border:1px solid var(--border);border-radius:8px;margin:20px 0;overflow:hidden}.playground__pane-header{background:var(--bg);color:var(--text-muted);text-transform:uppercase;letter-spacing:.04em;border-bottom:1px solid var(--border);justify-content:space-between;align-items:center;padding:6px 12px;font-size:12px;display:flex}.playground__reset-btn{text-transform:none;letter-spacing:normal;padding:2px 8px;font-size:11px}.playground__editor{font-family:var(--mono);background:var(--bg);min-height:120px;font-size:13px}.playground__error{background:var(--error-bg);color:var(--error-text);border-top:1px solid var(--error-border);font-family:var(--mono);white-space:pre-wrap;padding:10px 12px;font-size:12px;display:block}.playground__preview{background:var(--surface);border-top:1px solid var(--border);padding:16px;display:block}