:root {
  --color-bg: #0f172a;
  --color-surface: #111827;
  --color-panel: #1f2937;
  --color-text: #f8fafc;
  --color-subtle: #94a3b8;
  --color-accent: #3b82f6;
  --color-accent-soft: rgba(59, 130, 246, 0.15);
  --color-success: #22c55e;
  --color-warning: #f59e0b;
  --color-border: rgba(148, 163, 184, 0.2);
  --font-family: 'Segoe UI', sans-serif;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: var(--font-family);
  background: var(--color-bg);
  color: var(--color-text);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

h1, h2, h3, h4 {
  margin: 0;
  font-weight: 600;
}

p {
  margin: 0 0 0.5rem;
}

.app-header {
  background: linear-gradient(120deg, rgba(59, 130, 246, 0.2), rgba(14, 116, 144, 0.35));
  padding: 1.25rem 2rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
  border-bottom: 1px solid var(--color-border);
}

.brand {
  display: flex;
  align-items: center;
  gap: 1rem;
}

.brand__logo {
  display: inline-flex;
  width: 48px;
  height: 48px;
  border-radius: 12px;
  border: 2px solid var(--color-accent);
  align-items: center;
  justify-content: center;
  color: var(--color-accent);
  font-weight: 700;
}

.brand h1 {
  font-size: 1.6rem;
}

.brand p {
  color: var(--color-subtle);
}

.progress-summary {
  font-size: 0.95rem;
  color: var(--color-subtle);
}

.app-shell {
  flex: 1;
  display: flex;
  min-height: 0;
}

.sidebar {
  width: 260px;
  background: var(--color-surface);
  padding: 1.5rem 1rem;
  border-right: 1px solid var(--color-border);
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.sidebar h2 {
  font-size: 1.1rem;
  margin-bottom: 0.5rem;
}

.course-list {
  list-style: none;
  padding: 0;
  margin: 0;
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.course-list button {
  width: 100%;
  padding: 0.75rem 1rem;
  border: 1px solid transparent;
  border-radius: 10px;
  background: transparent;
  color: inherit;
  text-align: left;
  cursor: pointer;
  transition: background 0.2s, border-color 0.2s;
}

.course-list button:hover,
.course-list button[aria-pressed="true"] {
  background: var(--color-accent-soft);
  border-color: var(--color-accent);
}

.sidebar__footer {
  margin-top: auto;
}

.ghost-button {
  width: 100%;
  padding: 0.6rem 0.8rem;
  border: 1px solid var(--color-border);
  border-radius: 8px;
  background: transparent;
  color: var(--color-subtle);
  cursor: pointer;
  transition: border-color 0.2s, color 0.2s;
}

.ghost-button:hover {
  border-color: var(--color-accent);
  color: var(--color-text);
}

.content {
  flex: 1;
  display: flex;
  flex-direction: column;
  padding: 1.5rem;
  gap: 1.5rem;
  overflow-y: auto;
}

.lesson-selector {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: 14px;
  padding: 1.25rem;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.lesson-selector__header h2 {
  font-size: 1.3rem;
}

.lesson-selector__header p {
  color: var(--color-subtle);
}

.lesson-list {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 0.75rem;
  list-style: none;
  padding: 0;
  margin: 0;
}

.lesson-list button {
  width: 100%;
  border: 1px solid var(--color-border);
  border-radius: 12px;
  background: var(--color-panel);
  color: inherit;
  padding: 0.9rem;
  text-align: left;
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
  cursor: pointer;
  transition: border-color 0.2s, transform 0.1s;
}

.lesson-list button:hover,
.lesson-list button[aria-pressed="true"] {
  border-color: var(--color-accent);
  transform: translateY(-1px);
}

.lesson-list .lesson-meta {
  display: flex;
  align-items: center;
  justify-content: space-between;
  font-size: 0.85rem;
  color: var(--color-subtle);
}

.lesson-progress {
  display: inline-flex;
  align-items: center;
  gap: 0.25rem;
  font-weight: 600;
}

.lesson-progress[data-status="completed"] {
  color: var(--color-success);
}

.lesson-container {
  display: grid;
  grid-template-columns: minmax(280px, 400px) 1fr;
  gap: 1.5rem;
  align-items: start;
}

.lesson-details {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: 14px;
  padding: 1.5rem;
  display: flex;
  flex-direction: column;
  gap: 1rem;
  min-height: 320px;
}

.lesson-overview {
  color: var(--color-subtle);
}

.lesson-section h3 {
  font-size: 1rem;
  margin-bottom: 0.5rem;
}

.lesson-section ul,
.lesson-section ol {
  margin: 0;
  padding-left: 1.25rem;
  color: var(--color-subtle);
}

.lesson-explanation p {
  color: var(--color-subtle);
  margin-bottom: 0.75rem;
}

.lesson-example {
  margin: 0;
  padding: 1rem;
  background: #0b1120;
  border-radius: 10px;
  border: 1px solid var(--color-border);
  font-family: 'Fira Code', 'Consolas', 'Courier New', monospace;
  font-size: 0.9rem;
  white-space: pre;
  overflow-x: auto;
}

.lesson-section a {
  color: var(--color-accent);
  text-decoration: none;
}

.lesson-section a:hover {
  text-decoration: underline;
}

.lab-panel {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: 14px;
  padding: 1.25rem;
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
}

.lab-panel__header {
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.lab-actions {
  display: flex;
  gap: 0.5rem;
}

.primary-button,
.secondary-button {
  padding: 0.55rem 0.9rem;
  border-radius: 8px;
  border: none;
  font-weight: 600;
  cursor: pointer;
  transition: transform 0.1s, box-shadow 0.2s;
}

.primary-button {
  background: var(--color-accent);
  color: #fff;
  box-shadow: 0 8px 20px rgba(59, 130, 246, 0.25);
}

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

.secondary-button {
  background: transparent;
  color: var(--color-accent);
  border: 1px solid rgba(59, 130, 246, 0.6);
}

.secondary-button:hover {
  background: var(--color-accent-soft);
}

.code-editor {
  width: 100%;
  min-height: 220px;
  border-radius: 10px;
  border: 1px solid var(--color-border);
  background: #0b1120;
  color: #e2e8f0;
  font-family: 'Fira Code', 'Consolas', 'Courier New', monospace;
  font-size: 0.95rem;
  padding: 1rem;
  resize: vertical;
}

.lab-output {
  background: #0b1120;
  border-radius: 10px;
  border: 1px solid var(--color-border);
  padding: 1rem;
}

.lab-output h4 {
  margin-bottom: 0.5rem;
}

.console-output {
  margin: 0;
  white-space: pre-wrap;
  color: var(--color-subtle);
}

.checks-panel {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: 14px;
  padding: 1.25rem;
  grid-column: span 2;
}

.checks-list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
}

.check-item {
  border: 1px solid var(--color-border);
  border-radius: 12px;
  padding: 0.9rem;
  background: var(--color-panel);
}

.check-item__body {
  display: flex;
  gap: 0.75rem;
  align-items: flex-start;
}

.check-item__status {
  width: 1.75rem;
  height: 1.75rem;
  border-radius: 50%;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  font-size: 1rem;
  background: rgba(148, 163, 184, 0.2);
  border: 1px solid rgba(148, 163, 184, 0.3);
  flex-shrink: 0;
}

.check-item__status[data-state="pending"] {
  color: var(--color-subtle);
}

.check-item__status[data-state="passed"] {
  background: rgba(34, 197, 94, 0.18);
  border-color: rgba(34, 197, 94, 0.4);
  color: var(--color-success);
}

.check-item__status[data-state="failed"] {
  background: rgba(244, 63, 94, 0.18);
  border-color: rgba(244, 63, 94, 0.4);
  color: #f87171;
}

.check-item__description {
  font-weight: 500;
  margin: 0 0 0.4rem;
}

.check-item__actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
}

.check-item__actions button {
  padding: 0.4rem 0.7rem;
  border-radius: 6px;
  border: 1px solid rgba(148, 163, 184, 0.4);
  background: transparent;
  color: var(--color-subtle);
  cursor: pointer;
  transition: border-color 0.2s, color 0.2s;
}

.check-item__actions button:hover {
  border-color: var(--color-accent);
  color: var(--color-text);
}

.check-item__feedback {
  margin: 0.5rem 0 0;
  font-size: 0.85rem;
  color: var(--color-subtle);
}

@media (max-width: 1100px) {
  .lesson-container {
    grid-template-columns: 1fr;
  }

  .checks-panel {
    grid-column: auto;
  }
}

@media (max-width: 768px) {
  .app-shell {
    flex-direction: column;
  }

  .sidebar {
    width: auto;
    flex-direction: row;
    align-items: flex-start;
    overflow-x: auto;
  }

  .sidebar nav {
    flex: 1;
  }

  .course-list {
    flex-direction: row;
  }

  .lesson-selector {
    padding: 1rem;
  }
}