:root {
  --paper: #F6F4FC;
  --ink: #241B4D;
  --ink-soft: #5C5586;
  --accent: #6C5CE7;
  --accent-dark: #5645C9;
  --accent-soft: #C7BEFF;
  --rule: #E3DEF7;
  --err: #B3441E;
  --card-bg: #FFFFFF;
  --accent-rgb: 108, 92, 231;

  /* The worked-problem panel is a fixed brand signature — same dark
     indigo in both light and dark site themes, so these are NOT
     overridden below. */
  --panel-bg: #241B4D;
  --panel-text: #DCE6F5;
  --panel-tag: #C7BEFF;

  --display: 'Fraunces', Georgia, serif;
  --body: 'Inter', -apple-system, sans-serif;
  --mono: 'IBM Plex Mono', 'Courier New', monospace;
}

html[data-theme="dark"] {
  --paper: #14102A;
  --ink: #ECE9FB;
  --ink-soft: #A79FCF;
  --accent: #9B8CFF;
  --accent-dark: #B3A6FF;
  --accent-soft: #4B3F82;
  --rule: #2C2550;
  --err: #FF8A6B;
  --card-bg: #1D1840;
  --accent-rgb: 155, 140, 255;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }

body {
  margin: 0;
  background: var(--paper);
  color: var(--ink);
  font-family: var(--body);
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
  transition: background 0.2s ease, color 0.2s ease;
}

a { color: inherit; }

.wrap {
  max-width: 1280px;
  margin: 0 auto;
  padding: 0 40px;
}
@media (min-width: 640px) {
  .wrap { padding: 0 56px; }
}
@media (min-width: 1000px) {
  .wrap { padding: 0 72px; }
}
@media (min-width: 1600px) {
  .wrap { padding: 0 96px; }
}

/* ---------- Header ---------- */
/* Top/bottom breathing room scales with viewport the same way the .wrap
   side padding does, so the page never feels flush against the edge on
   any screen size. */
header {
  padding-top: 40px;
}
@media (min-width: 640px) {
  header { padding-top: 56px; }
}
@media (min-width: 1000px) {
  header { padding-top: 72px; }
}
@media (min-width: 1600px) {
  header { padding-top: 96px; }
}
.header-row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  flex-wrap: wrap;
}
.logo {
  font-family: var(--display);
  font-size: 26px;
  letter-spacing: -0.01em;
  font-weight: 600;
  color: var(--ink);
  text-decoration: none;
}

.nav-controls {
  display: flex;
  align-items: center;
  gap: 12px;
  flex-wrap: wrap;
}

.icon-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  font-family: var(--body);
  font-size: 14px;
  font-weight: 600;
  color: var(--ink);
  background: transparent;
  border: 1px solid var(--rule);
  border-radius: 999px;
  padding: 9px 15px;
  cursor: pointer;
  text-decoration: none;
  line-height: 1;
  transition: border-color 0.15s ease, color 0.15s ease;
}
.icon-btn:hover { border-color: var(--accent); color: var(--accent); }
.icon-btn:focus-visible { outline: 3px solid var(--ink); outline-offset: 2px; }
#themeToggle { font-size: 17px; padding: 9px 12px; }

/* Primary CTA style — used for the nav button, hero button and submit
   button, so the action always looks the same and is easy to find. */
.cta {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 10px;
  font-family: var(--body);
  font-weight: 700;
  font-size: 19px;
  text-decoration: none;
  background: var(--accent);
  color: #fff;
  padding: 20px 36px;
  border-radius: 8px;
  border: none;
  cursor: pointer;
  box-shadow: 0 10px 26px rgba(var(--accent-rgb), 0.42);
  transition: background 0.15s ease, transform 0.15s ease, box-shadow 0.15s ease;
  max-width: 100%;
}
.cta:hover {
  background: var(--accent-dark);
  transform: translateY(-2px) scale(1.02);
  box-shadow: 0 14px 32px rgba(var(--accent-rgb), 0.5);
}
.cta:focus-visible { outline: 3px solid var(--ink); outline-offset: 3px; }
nav .cta { font-size: 16px; padding: 15px 26px; box-shadow: 0 6px 16px rgba(var(--accent-rgb), 0.34); }
.hero .cta { font-size: 16px; padding: 15px 28px; box-shadow: 0 6px 16px rgba(var(--accent-rgb), 0.34); }

@media (max-width: 480px) {
  .cta { font-size: 15px; }
  .hero .cta { width: 100%; padding: 15px 20px; }
  nav .cta { padding: 12px 18px; }
}

/* ---------- Hero ---------- */
.hero {
  padding-top: 96px;
  padding-bottom: 56px;
}
.eyebrow {
  display: inline-block;
  font-family: var(--mono);
  font-size: 12.5px;
  color: var(--accent-dark);
  letter-spacing: 0.04em;
  margin: 0 0 20px;
  text-decoration: none;
  border-bottom: 1px solid transparent;
  transition: border-color 0.15s ease;
}
.eyebrow:hover { border-color: var(--accent-dark); }
h1 {
  font-family: var(--display);
  font-weight: 500;
  font-size: clamp(32px, 5.2vw, 56px);
  line-height: 1.1;
  letter-spacing: -0.01em;
  margin: 0 0 24px;
  max-width: 20ch;
}
h1 em {
  font-style: italic;
  font-weight: 500;
  color: var(--accent);
}
.sub {
  font-size: 17px;
  color: var(--ink-soft);
  max-width: 46ch;
  margin: 0 0 36px;
}

/* ---------- Worked-problem panel (signature element) ---------- */
.proof-panels {
  display: flex;
  flex-wrap: wrap;
  gap: 28px;
  margin-top: 56px;
}
.proof-panel {
  background: var(--panel-bg);
  color: var(--panel-text);
  border-radius: 6px;
  padding: 32px 32px 28px;
  font-family: var(--mono);
  font-size: 14px;
  flex: 1 1 460px;
  max-width: 520px;
}
.proof-label {
  font-size: 11.5px;
  letter-spacing: 0.06em;
  color: var(--panel-tag);
  margin-bottom: 14px;
  opacity: 0.9;
}
.proof-line {
  opacity: 0;
  transform: translateY(4px);
  margin: 0 0 9px;
  transition: opacity 0.4s ease, transform 0.4s ease;
}
.proof-line.show { opacity: 1; transform: translateY(0); }
.proof-line .tag { color: var(--panel-tag); margin-right: 8px; }

@media (prefers-reduced-motion: reduce) {
  .proof-line { transition: none; opacity: 1; transform: none; }
}

/* ---------- How it works ---------- */
.how {
  padding-top: 80px;
  padding-bottom: 32px;
  border-top: 1px solid var(--rule);
}
.how-label {
  font-family: var(--mono);
  font-size: 12.5px;
  color: var(--ink-soft);
  margin: 0 0 24px;
}
.how-list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: 18px;
}
.how-list li {
  display: flex;
  gap: 16px;
  align-items: baseline;
}
.how-list .num {
  font-family: var(--mono);
  font-size: 13px;
  color: var(--accent);
  flex: none;
  width: 22px;
}
.how-list p { margin: 0; font-size: 15px; }
.how-list strong { font-weight: 600; }
.how-list span.desc { color: var(--ink-soft); }

.how-columns {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 56px;
}
.how-col h3 {
  font-family: var(--display);
  font-weight: 500;
  font-size: 17px;
  margin: 0 0 18px;
  color: var(--ink);
}

@media (max-width: 620px) {
  .how-columns { grid-template-columns: 1fr; gap: 40px; }
}

/* ---------- Contact (footer) ---------- */
.contact-list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  align-items: flex-end;
  gap: 8px;
}
.contact-list a {
  font-family: var(--mono);
  font-size: 13.5px;
  text-decoration: none;
  color: var(--ink-soft);
  border-bottom: 1px solid transparent;
  transition: color 0.15s ease, border-color 0.15s ease;
}
.contact-list a:hover { color: var(--accent); border-color: var(--accent); }

/* ---------- Form ---------- */
.signup {
  padding-top: 88px;
  padding-bottom: 56px;
  border-top: 1px solid var(--rule);
}
.signup h2 {
  font-family: var(--display);
  font-weight: 500;
  font-size: clamp(24px, 3.6vw, 30px);
  margin: 0 0 10px;
}
.signup .sub { margin-bottom: 36px; }

form {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 20px;
  max-width: 720px;
}
.field { display: flex; flex-direction: column; gap: 7px; }
.field.full { grid-column: 1 / -1; }
label {
  font-size: 13px;
  font-weight: 500;
  color: var(--ink-soft);
}
input, textarea {
  font-family: var(--body);
  font-size: 15px;
  color: var(--ink);
  background: var(--card-bg);
  border: 1px solid var(--rule);
  border-radius: 4px;
  padding: 12px 14px;
  outline: none;
  transition: border-color 0.15s ease, background 0.15s ease;
}
input:focus, textarea:focus { border-color: var(--accent); }
textarea { resize: vertical; min-height: 88px; }

.submit-row {
  grid-column: 1 / -1;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 16px;
  margin-top: 16px;
}
.submit-row .cta { font-size: 21px; padding: 23px 44px; }
.submit-row .cta:disabled { opacity: 0.6; cursor: default; transform: none; }

.status { font-size: 14px; }
.status.ok { color: var(--accent-dark); font-weight: 600; }
.status.err { color: var(--err); }

@media (max-width: 620px) {
  form { grid-template-columns: 1fr; }
  .header-row { flex-wrap: wrap; }
}

@media (max-width: 480px) {
  .submit-row .cta { width: 100%; font-size: 17px; padding: 18px 26px; }
}

footer {
  padding-top: 64px;
  padding-bottom: 40px;
  border-top: 1px solid var(--rule);
  margin-top: 6px;
  font-family: var(--mono);
  font-size: 12.5px;
  color: var(--ink-soft);
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 24px;
  flex-wrap: wrap;
}
@media (min-width: 640px) {
  footer { padding-bottom: 56px; }
}
@media (min-width: 1000px) {
  footer { padding-bottom: 72px; }
}
@media (min-width: 1600px) {
  footer { padding-bottom: 96px; }
}
.logo-foot {
  font-family: var(--display);
  font-size: 15px;
  font-weight: 600;
  color: var(--ink);
  text-decoration: none;
}
.footer-left {
  display: flex;
  flex-direction: column;
  gap: 6px;
}
.copyright { color: var(--ink-soft); }

@media (max-width: 620px) {
  footer { flex-direction: column; align-items: flex-start; gap: 24px; }
  .contact-list { align-items: flex-start; }
}

:focus-visible { outline: 3px solid var(--ink); outline-offset: 2px; }

.visually-hidden {
  position: absolute;
  width: 1px; height: 1px;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}