:root {
  --question-ink: #18212f;
  --question-muted: #5d6878;
  --question-accent: #3157a4;
  --question-accent-dark: #203d79;
  --question-soft: #eef3fb;
  --question-line: #dce3ee;
}

body {
  color: var(--question-ink);
}

.navbar-brand {
  font-weight: 700;
  letter-spacing: -0.02em;
}

.sidebar-title {
  font-weight: 750;
}

h1, h2, h3 {
  letter-spacing: -0.025em;
}

.hero {
  max-width: 860px;
  margin: 1.25rem auto 3rem;
  padding: 3.5rem;
  border-radius: 1.5rem;
  background: linear-gradient(135deg, #eef3fb 0%, #f7f1e8 100%);
  font-size: 1.2rem;
  color: #344054;
  box-shadow: 0 18px 50px rgba(33, 52, 88, 0.08);
}

.hero .btn {
  margin-top: 1rem;
}

.btn-primary {
  background-color: var(--question-accent);
  border-color: var(--question-accent);
}

.topic-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 1rem;
  margin: 1.5rem 0 2.5rem;
}

.topic-card {
  padding: 1.4rem 1.5rem;
  border: 1px solid var(--question-line);
  border-radius: 1rem;
  background: #fff;
  box-shadow: 0 7px 22px rgba(33, 52, 88, 0.05);
}

.topic-card h3 {
  margin-top: 0;
  font-size: 1.15rem;
}

.topic-card p:last-child {
  margin-bottom: 0;
  color: var(--question-muted);
}

.home-note {
  max-width: 820px;
  margin: 0 auto;
  padding: 1rem 1.25rem;
  border-left: 4px solid var(--question-accent);
  background: var(--question-soft);
}

.preface {
  max-width: 900px;
  margin: 1.25rem 0 3rem;
  padding: 1.5rem 1.75rem;
  border: 1px solid var(--question-line);
  border-radius: 1rem;
  background: #fff;
  line-height: 1.75;
}

.top-language-switch {
  display: inline-flex;
  align-items: center;
  margin-right: 0.65rem;
  padding: 0.15rem;
  border: 1px solid var(--question-line);
  border-radius: 999px;
  background: #fff;
  overflow: hidden;
}

.top-language-option {
  padding: 0.28rem 0.65rem;
  border-radius: 999px;
  color: var(--question-muted);
  font-size: 0.9rem;
  font-weight: 700;
  line-height: 1.2;
  text-decoration: none;
  white-space: nowrap;
}

.top-language-option:hover {
  background: var(--question-soft);
  color: var(--question-accent-dark);
}

.top-language-option.active,
.top-language-option.active:hover {
  background: var(--question-accent);
  color: #fff;
  cursor: default;
}

.question-card {
  margin: 0.75rem 0 2.5rem;
  padding: 1.35rem 1.5rem;
  border: 1px solid var(--question-line);
  border-left: 5px solid var(--question-accent);
  border-radius: 0.75rem;
  background: #fff;
  box-shadow: 0 7px 20px rgba(33, 52, 88, 0.05);
  font-size: 1.04rem;
}

.question-card > p:first-child {
  font-weight: 550;
}

.question-card details {
  margin-top: 1.1rem;
  border-top: 1px solid var(--question-line);
}

.question-card summary {
  width: fit-content;
  margin-top: 1rem;
  padding: 0.55rem 0.9rem;
  border-radius: 0.55rem;
  background: var(--question-accent);
  color: white;
  cursor: pointer;
  font-weight: 650;
  user-select: none;
}

.question-card summary:hover {
  background: var(--question-accent-dark);
}

.question-card details[open] summary {
  margin-bottom: 1rem;
}

@media (max-width: 700px) {
  .hero {
    padding: 2rem 1.4rem;
  }

  .topic-grid {
    grid-template-columns: 1fr;
  }
}