:root {
  --color-bg: #f4f7fb;
  --color-surface: #ffffff;
  --color-text: #1f2937;
  --color-muted: #4b5563;
  --color-primary: #1d4ed8;
  --color-primary-dark: #1e3a8a;
  --color-border: #dbe3ef;
  --color-correct: #15803d;
  --color-correct-bg: #ecfdf3;
  --color-incorrect: #b91c1c;
  --color-incorrect-bg: #fef2f2;
  --shadow: 0 10px 30px rgba(15, 23, 42, 0.08);
  --radius: 12px;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
  line-height: 1.65;
  color: var(--color-text);
  background: linear-gradient(180deg, #e8eef8 0%, var(--color-bg) 220px);
}

.page-header,
.content,
.quiz-section {
  max-width: 820px;
  margin: 0 auto;
  padding: 0 1.5rem;
}

.page-header {
  padding-top: 2.5rem;
  padding-bottom: 1rem;
}

.page-header h1 {
  margin: 0 0 0.5rem;
  font-size: clamp(1.75rem, 4vw, 2.35rem);
  line-height: 1.2;
  color: var(--color-primary-dark);
}

.page-header p {
  margin: 0;
  color: var(--color-muted);
}

.content {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 2rem 1.75rem;
}

.content h2 {
  margin: 2rem 0 0.75rem;
  font-size: 1.35rem;
  color: var(--color-primary-dark);
  border-bottom: 2px solid #e5edf9;
  padding-bottom: 0.35rem;
}

.content h2:first-child {
  margin-top: 0;
}

.content h3 {
  margin: 0 0 0.75rem;
  font-size: 1.05rem;
  color: var(--color-text);
}

.content p {
  margin: 0 0 1rem;
}

.content ul {
  margin: 0 0 1rem;
  padding-left: 1.4rem;
}

.content li {
  margin-bottom: 0.35rem;
}

.quiz-section {
  margin-top: 2rem;
  margin-bottom: 3rem;
}

.quiz-intro {
  margin-bottom: 1.5rem;
}

.quiz-intro h2 {
  margin: 0 0 0.5rem;
  font-size: 1.5rem;
  color: var(--color-primary-dark);
}

.quiz-intro p {
  margin: 0;
  color: var(--color-muted);
}

.quiz-question {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.5rem;
  margin-bottom: 1.25rem;
}

.quiz-question.is-answered {
  border-color: #c7d7f5;
}

.quiz-options {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 0.75rem;
}

.quiz-option {
  width: 100%;
  display: flex;
  align-items: flex-start;
  gap: 0.75rem;
  text-align: left;
  padding: 0.9rem 1rem;
  border: 1px solid var(--color-border);
  border-radius: 10px;
  background: #fafcff;
  color: inherit;
  font: inherit;
  cursor: pointer;
  transition: border-color 0.15s ease, background-color 0.15s ease, transform 0.15s ease;
}

.quiz-option:hover:not(:disabled) {
  border-color: #93b4f4;
  background: #f5f8ff;
  transform: translateY(-1px);
}

.quiz-option:disabled {
  cursor: default;
}

.quiz-option-label {
  flex-shrink: 0;
  font-weight: 700;
  color: var(--color-primary);
}

.quiz-option.is-selected.is-correct {
  border-color: var(--color-correct);
  background: var(--color-correct-bg);
}

.quiz-option.is-selected.is-incorrect {
  border-color: var(--color-incorrect);
  background: var(--color-incorrect-bg);
}

.quiz-option.is-reveal-correct:not(.is-selected) {
  border-color: var(--color-correct);
  background: var(--color-correct-bg);
}

.quiz-explanation {
  display: none;
  margin-top: 0.75rem;
  padding: 0.9rem 1rem;
  border-radius: 10px;
  background: #f8fafc;
  border-left: 4px solid var(--color-primary);
}

.quiz-explanation.is-visible {
  display: block;
}

.quiz-status {
  margin: 0 0 0.5rem;
  font-weight: 700;
}

.quiz-status.is-correct {
  color: var(--color-correct);
}

.quiz-status.is-incorrect {
  color: var(--color-incorrect);
}

.quiz-explanation p:last-child {
  margin-bottom: 0;
}

.quiz-summary {
  display: none;
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.25rem 1.5rem;
}

.quiz-summary.is-visible {
  display: block;
}

.quiz-summary h3 {
  margin: 0 0 0.5rem;
  color: var(--color-primary-dark);
}

.quiz-summary p {
  margin: 0;
}

@media (max-width: 640px) {
  .content,
  .quiz-question {
    padding: 1.25rem 1rem;
  }
}