:root {
  color-scheme: light;
  font-family: Inter, system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  background: #f4f4f4;
  color: #101010;
}

* {
  box-sizing: border-box;
}

html, body {
  margin: 0;
  min-height: 100%;
  background: #f4f4f4;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 24px;
}

button {
  font: inherit;
}

.app-shell {
  width: min(100%, 840px);
}

.screen {
  position: relative;
  width: 100%;
  padding: 32px;
  background: #ffffff;
  border: 2px solid #000;
  border-radius: 20px;
  box-shadow: 0 18px 40px rgba(0, 0, 0, 0.08);
}

.hidden {
  display: none;
}

.card {
  display: grid;
  gap: 20px;
  align-items: center;
}

h1 {
  margin: 0;
  font-size: clamp(1.6rem, 2.2vw, 2.4rem);
  line-height: 1.1;
  letter-spacing: -0.02em;
  text-align: center;
}

.subtitle {
  margin: 0;
  color: #444;
  line-height: 1.5;
  text-align: center;
}

.button-group,
.button-row {
  display: flex;
  flex-wrap: wrap;
  gap: 16px;
}

.button-group {
  flex-direction: column;
  align-items: center;
}

.button-row {
  justify-content: space-between;
}

.button-stack {
  display: flex;
  flex-direction: column;
  gap: 16px;
  width: 100%;
}

.button-help-row {
  display: flex;
  justify-content: center;
  width: 100%;
}

.button-nav-row {
  display: flex;
  justify-content: space-between;
  width: 100%;
  gap: 16px;
}

.button-nav-row .btn {
  flex: 1;
}

.btn {
  border: none;
  border-radius: 12px;
  padding: 16px 24px;
  cursor: pointer;
  font-size: 1rem;
  font-weight: 600;
  transition: transform 0.2s ease, filter 0.2s ease;
  width: 100%;
}

.btn:hover {
  transform: translateY(-1px);
}

.btn-primary {
  background: #101010;
  color: #fff;
}

.btn-secondary {
  background: #fff;
  color: #101010;
  border: 1px solid #101010;
}

.btn-help {
  background: #1d4ed8;
  color: #fff;
}

.checklist {
  list-style: none;
  padding: 0;
  margin: 0;
  display: grid;
  gap: 14px;
}

.checklist li {
  background: #fafafa;
  border: 1px solid #ddd;
  border-radius: 14px;
  padding: 16px;
}

.checklist label {
  display: flex;
  align-items: center;
  gap: 12px;
  cursor: pointer;
  font-size: 1rem;
}

.vocab-list {
  list-style: none;
  padding: 0;
  margin: 0;
  display: grid;
  gap: 14px;
}

.vocab-list li {
  background: #fafafa;
  border: 1px solid #ddd;
  border-radius: 14px;
  padding: 16px;
  line-height: 1.5;
}

.checklist input[type='checkbox'] {
  width: 20px;
  height: 20px;
}

.content-box {
  background: #f7f7f7;
  border-radius: 18px;
  padding: 20px;
  border: 1px solid #e0e0e0;
}

.video-card {
  display: grid;
  gap: 16px;
  padding: 20px;
  background: #000;
  color: #fff;
  text-decoration: none;
  border-radius: 18px;
  align-items: center;
}

.video-thumb {
  width: 100%;
  min-height: 200px;
  background: linear-gradient(135deg, #222, #000);
  border-radius: 18px;
  display: grid;
  place-items: center;
}

.play-icon {
  font-size: 2.5rem;
}

.card-certificate {
  text-align: center;
}

.tutorial-image {
  width: 100%;
  max-width: 500px;
  height: auto;
  border-radius: 12px;
  object-fit: cover;
  margin: 16px auto;
  display: block;
}

.certificate {
  margin: 0 auto;
  max-width: 620px;
  border: 2px solid #101010;
  border-radius: 20px;
  padding: 28px;
  background: #fff;
}

.certificate-label {
  font-size: 1.1rem;
  font-weight: 700;
  margin-bottom: 18px;
}

.certificate-date {
  margin-top: 24px;
  font-weight: 700;
}

@media (max-width: 640px) {
  .screen {
    padding: 20px;
  }

  .button-row,
  .button-group {
    gap: 12px;
  }

  .button-row {
    flex-direction: column;
  }
}