:root { --ink:#1B1B2F; --sub:#4A4A63; --head:#7C00FF; --head-d:#5A00BA; --tint:#F4EEFF; --good:#0B8A42;
  --bad:#C62828; --panel:#F7F7FB; --line:#E2E2EC; --gold:#C9A227; --bg:#FFFFFF; --radius:14px;
  --shadow:0 1px 2px rgba(27,27,47,.06), 0 6px 20px rgba(27,27,47,.06); }
* { box-sizing:border-box; }
html { scroll-behavior:smooth; }
body { margin:0; background:var(--bg); color:var(--ink); font:17px/1.7 "Open Sans", system-ui, sans-serif;
  -webkit-font-smoothing:antialiased; }
img { max-width:100%; height:auto; display:block; }
a { color:var(--head); text-underline-offset:3px; }
a:hover { color:var(--head-d); }
.wrap { max-width:1080px; margin:0 auto; padding:0 20px; }
.narrow { max-width:780px; }
/* One typeface for all text (user, About page, 2026-10-03); Montserrat only on the
   brand wordmark and tagline, as in the videos. */
h1, h2, h3 { font-family:"Open Sans", sans-serif; line-height:1.25; color:var(--ink); }
h1 { font-size:clamp(30px, 5vw, 46px); font-weight:800; margin:0 0 14px; letter-spacing:-.01em; }
h2 { font-size:clamp(22px, 3vw, 28px); font-weight:700; margin:0 0 12px; }
h3 { font-size:19px; font-weight:700; margin:0 0 8px; }
p { margin:0 0 14px; }
.eyebrow { font:700 13px "Open Sans", sans-serif; letter-spacing:.08em; text-transform:uppercase; color:var(--head); margin:0 0 10px; }
.lead { font-size:19px; color:var(--sub); max-width:700px; }
.muted { color:var(--sub); font-size:15px; }

/* header */
.site-header { position:sticky; top:0; z-index:10; background:rgba(255,255,255,.94); backdrop-filter:blur(8px);
  border-bottom:1px solid var(--line); }
.site-header .wrap { display:flex; align-items:center; gap:14px; min-height:72px; }
.brand-link { display:flex; align-items:center; gap:12px; text-decoration:none; }
.brand-link img { width:48px; height:48px; }
.brand-text { display:flex; flex-direction:column; gap:3px; padding-top:4px; }
.brand-name { font:800 22px/1 Montserrat, sans-serif; color:var(--head); }
.brand-tag { font:600 12px Montserrat, sans-serif; color:var(--sub); }
.main-nav { display:flex; gap:4px; margin-left:18px; }
.main-nav a { font:600 15px "Open Sans", sans-serif; color:var(--ink); text-decoration:none; padding:8px 12px; border-radius:8px; }
.main-nav a:hover, .main-nav a[aria-current] { background:var(--tint); color:var(--head); }
.social { margin-left:auto; display:flex; align-items:center; gap:8px; }
.social a, .social button { display:inline-flex; align-items:center; justify-content:center; gap:6px; height:40px;
  min-width:40px; padding:0 10px; border-radius:10px; border:1px solid var(--line); background:#fff; color:var(--ink);
  font:600 14px "Open Sans", sans-serif; text-decoration:none; cursor:pointer; transition:border-color .15s, box-shadow .15s; }
.social a:hover, .social button:hover { border-color:var(--head); box-shadow:0 2px 8px rgba(124,0,255,.12); }
.social svg { width:22px; height:22px; flex:none; }
.social button { color:var(--head); }

/* buttons */
.btns { display:flex; flex-wrap:wrap; gap:12px; margin-top:18px; }
.btn { display:inline-flex; align-items:center; gap:8px; padding:12px 20px; border-radius:10px;
  font:700 15px "Open Sans", sans-serif; text-decoration:none; border:2px solid var(--head); transition:transform .15s, box-shadow .15s; }
.btn:hover { transform:translateY(-1px); box-shadow:0 6px 16px rgba(124,0,255,.18); }
.btn svg { width:18px; height:18px; }
.btn.primary { background:var(--head); color:#fff; }
.btn.primary:hover { background:var(--head-d); color:#fff; }
.btn.ghost { background:#fff; color:var(--head); }
.btn.small { padding:9px 14px; font-size:14px; }

/* hero */
.hero { background:radial-gradient(1200px 400px at 85% -10%, #EADCFF 0%, transparent 60%),
  linear-gradient(180deg, #FBF8FF 0%, #FFFFFF 100%); border-bottom:1px solid var(--line); }
.hero .wrap { display:grid; grid-template-columns:1.15fr .85fr; gap:40px; align-items:center; padding-top:56px; padding-bottom:56px; }
.hero-card { background:#fff; border:1px solid var(--line); border-radius:18px; box-shadow:var(--shadow); padding:18px; }
.hero-card figcaption { font-size:14px; color:var(--sub); margin-top:8px; text-align:center; }
.page-head { background:linear-gradient(180deg, #FBF8FF 0%, #FFFFFF 100%); border-bottom:1px solid var(--line); padding:34px 0 30px; }

/* sections */
section.block { padding:52px 0; }
section.block.alt { background:var(--panel); border-top:1px solid var(--line); border-bottom:1px solid var(--line); }
.grid { display:grid; gap:20px; }
.grid.three { grid-template-columns:repeat(3, 1fr); }
.grid.two { grid-template-columns:repeat(2, 1fr); }
.card { background:#fff; border:1px solid var(--line); border-radius:var(--radius); padding:22px 24px; box-shadow:var(--shadow); }
.step .num { display:inline-flex; width:36px; height:36px; border-radius:50%; align-items:center; justify-content:center;
  background:var(--tint); color:var(--head); font:800 16px "Open Sans", sans-serif; margin-bottom:10px; }
.chapter-card { display:block; text-decoration:none; color:var(--ink); transition:transform .15s, box-shadow .15s; }
.chapter-card:hover { transform:translateY(-2px); box-shadow:0 10px 28px rgba(27,27,47,.10); color:var(--ink); }
.chip { display:inline-block; font:700 12px "Open Sans", sans-serif; color:var(--head); background:var(--tint);
  border-radius:999px; padding:4px 10px; margin-right:6px; }
.lesson-card { display:grid; grid-template-columns:240px 1fr; gap:24px; align-items:center; }
.lesson-card .thumb { background:#fff; border:1px solid var(--line); border-radius:12px; padding:10px; }
.lesson-card h2 { font-size:22px; }
.lesson-card h2 a { color:var(--ink); text-decoration:none; }
.lesson-card h2 a:hover { color:var(--head); }

/* breadcrumbs */
.crumbs { font-size:14px; color:var(--sub); margin:0 0 14px; }
.crumbs a { color:var(--sub); }
.crumbs span[aria-hidden] { margin:0 6px; color:#B8B8C8; }

/* lesson article */
.lesson-meta { display:flex; flex-wrap:wrap; gap:8px; margin:0 0 6px; }
article.lesson section { margin:0 0 34px; }
article.lesson h2 { padding-bottom:8px; border-bottom:2px solid var(--tint); margin-top:8px; }
article.lesson figure { margin:18px auto 22px; max-width:560px; text-align:center; }
article.lesson figure img { border-radius:10px; max-height:400px; width:auto; margin:0 auto; }
article.lesson figcaption { font-size:14px; color:var(--sub); text-align:center; margin-top:6px; }
.math { overflow-x:auto; overflow-y:hidden; }
.box { border-radius:var(--radius); padding:18px 22px; margin:20px 0; border:1px solid; }
.box.formula { background:#F6F0FF; border-color:#D9C2FF; }
.box.careful { background:#FFF3F3; border-color:#F3C4C4; }
.box.careful h3 { color:var(--bad); }
.box.example { background:#F0FAF4; border-color:#BFE5CD; }
.box.example h3 { color:var(--good); }
.box figure { max-width:420px; }
.answer { display:inline-block; background:#fff; border:1px solid #BFE5CD; border-radius:10px; padding:8px 14px; font-weight:600; }
table.quad { border-collapse:collapse; margin:6px 0 14px; background:#fff; border-radius:10px; overflow:hidden; }
table.quad th, table.quad td { border:1px solid var(--line); padding:8px 16px; text-align:left; }
table.quad th { background:#EFE6FF; font-family:"Open Sans", sans-serif; font-size:14px; }
ol.practice { padding-left:22px; }
ol.practice li { margin:0 0 14px; }
ol.practice li p { margin:0 0 6px; }
details { background:var(--panel); border:1px solid var(--line); border-radius:10px; padding:6px 12px; }
details summary { cursor:pointer; font:700 14px "Open Sans", sans-serif; color:var(--head); }
details[open] summary { margin-bottom:6px; }
.notify { display:flex; gap:20px; align-items:center; justify-content:space-between; flex-wrap:wrap;
  background:linear-gradient(120deg, #F4EEFF, #FFFFFF); border:1px solid #D9C2FF; }
.notify h2 { font-size:22px; margin:0 0 4px; }

/* about */
ul.for-list { padding-left:20px; }
ul.for-list li { margin:0 0 10px; }
.join { display:grid; grid-template-columns:repeat(3, 1fr); gap:16px; margin-top:16px; }
.join-card { display:flex; align-items:center; gap:14px; padding:18px; border:1px solid var(--line); border-radius:var(--radius);
  text-decoration:none; color:var(--ink); background:#fff; box-shadow:var(--shadow); transition:transform .15s, border-color .15s; }
.join-card:hover { transform:translateY(-2px); border-color:var(--head); color:var(--ink); }
.join-card svg { width:34px; height:34px; flex:none; }
.join-card span { display:flex; flex-direction:column; line-height:1.3; }
.join-card small { color:var(--sub); font-size:14px; word-break:break-all; }

/* footer */
.site-footer { background:#1B1B2F; color:#CFCFE0; margin-top:24px; padding:44px 0 26px; font-size:15px; }
.site-footer a { color:#fff; text-decoration:none; }
.site-footer a:hover { color:#D9C2FF; text-decoration:underline; }
.site-footer .cols { display:grid; grid-template-columns:1.4fr 1fr 1fr; gap:28px; }
.site-footer h3 { color:#fff; font-size:15px; letter-spacing:.06em; text-transform:uppercase; margin:0 0 10px; }
.site-footer ul { list-style:none; padding:0; margin:0; }
.site-footer li { margin:0 0 6px; }
.site-footer .fbrand { font:800 22px Montserrat, sans-serif; color:#fff; }
.site-footer .copy { border-top:1px solid #34344F; margin-top:30px; padding-top:16px; font-size:13px; color:#9A9AB5; }

.toast { position:fixed; left:50%; bottom:24px; transform:translateX(-50%); background:var(--ink); color:#fff;
  padding:10px 16px; border-radius:8px; font-size:14px; opacity:0; transition:opacity .2s; pointer-events:none; z-index:20; }
.toast.on { opacity:1; }
:focus-visible { outline:3px solid #B48CFF; outline-offset:2px; }

@media (max-width:860px) {
  .hero .wrap { grid-template-columns:1fr; padding-top:36px; padding-bottom:40px; }
  .grid.three, .grid.two, .join, .site-footer .cols { grid-template-columns:1fr; }
  .lesson-card { grid-template-columns:1fr; }
  .lesson-card .thumb { max-width:320px; }
}
@media (max-width:640px) {
  body { font-size:16px; }
  .site-header .wrap { flex-wrap:wrap; gap:8px 10px; min-height:0; padding-top:10px; padding-bottom:8px; }
  .main-nav { order:3; width:100%; margin:0; justify-content:flex-start; }
  .main-nav a { padding:6px 10px; font-size:14px; }
  .brand-link img { width:42px; height:42px; }
  .brand-tag { font-size:11px; }
  .social { gap:6px; } .social .label { display:none; } .social a, .social button { padding:0 8px; }
  section.block { padding:36px 0; }
  .lead { font-size:17px; }
}
@media (max-width:380px) {
  .brand-tag { display:none; } .brand-text { padding-top:0; } .brand-link img { width:38px; height:38px; } .brand-name { font-size:19px; }
  .social { gap:4px; } .social a, .social button { height:36px; min-width:36px; padding:0 6px; }
}