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

:root {
  --bg: #0f172a;
  --bg-elevated: #111827;
  --bg-card: #020617;
  --accent: #38bdf8;
  --accent-soft: rgba(56, 189, 248, 0.1);
  --accent-strong: #0ea5e9;
  --text: #e5e7eb;
  --muted: #9ca3af;
  --danger: #f97373;
  --success: #4ade80;
  --border-subtle: rgba(148, 163, 184, 0.3);
  --radius-lg: 16px;
  --radius-md: 10px;
  --shadow-soft: 0 18px 45px rgba(15, 23, 42, 0.75);
  --font: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

html,
body {
  margin: 0;
  padding: 0;
  background: #001a33; /* dark blue */
  color: var(--text);
  font-family: var(--font);
  height: 100%;
}

/* General hidden helper */
.hidden {
  display: none !important;
}

/* LOGIN SCREEN */

.login-screen {
  position: fixed;
  inset: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  background: #001a33; /* same dark blue as body */
  z-index: 999;
  padding: 16px;
}

.login-card {
  width: 100%;
  max-width: 380px;
  background: rgba(15, 23, 42, 0.97);
  border-radius: 18px;
  padding: 20px 18px;
  border: 1px solid var(--border-subtle);
  box-shadow: 0 22px 55px rgba(15, 23, 42, 0.9);
}

.login-card h1 {
  margin: 0 0 6px 0;
  font-size: 1.3rem;
}

.login-subtitle {
  margin: 0 0 14px 0;
  font-size: 0.85rem;
  color: var(--muted);
}

.login-form .form-row {
  margin-bottom: 10px;
}

.login-form label {
  display: block;
  font-size: 0.85rem;
  color: var(--muted);
  margin-bottom: 3px;
}

.login-form input {
  width: 100%;
  padding: 8px 10px;
  border-radius: var(--radius-md);
  border: 1px solid var(--border-subtle);
  background: #020617;
  color: var(--text);
  font-size: 0.9rem;
}

.login-btn {
  width: 100%;
  margin-top: 8px;
}

.login-error {
  margin-top: 8px;
  font-size: 0.8rem;
  color: var(--danger);
  min-height: 1em;
}

.login-hint {
  margin-top: 8px;
  font-size: 0.78rem;
  color: var(--muted);
}

/* APP LAYOUT */

.app {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.app-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 14px 16px;
  border-bottom: 1px solid var(--border-subtle);
  background: rgba(15, 23, 42, 0.95);
  backdrop-filter: blur(16px);
}

.logo-title h1 {
  margin: 0;
  font-size: 1.3rem;
}

.logo-title p {
  margin: 4px 0 0;
  color: var(--muted);
  font-size: 0.85rem;
}

.app-main {
  display: grid;
  grid-template-columns: 280px minmax(0, 1fr);
  gap: 20px;
  padding: 12px;
  flex: 1;
}

.sidebar {
  background: rgba(15, 23, 42, 0.95);
  border-radius: var(--radius-lg);
  padding: 14px;
  border: 1px solid var(--border-subtle);
  box-shadow: var(--shadow-soft);
}

.sidebar h2 {
  font-size: 1.05rem;
  margin-top: 0;
}

.field-label {
  display: block;
  font-size: 0.9rem;
  margin-bottom: 4px;
  color: var(--muted);
}

#topic-filter {
  width: 100%;
  padding: 8px 10px;
  border-radius: var(--radius-md);
  border: 1px solid var(--border-subtle);
  background: #020617;
  color: var(--text);
}

.stats-card,
.help-card {
  margin-top: 14px;
  padding: 10px;
  border-radius: var(--radius-md);
  background: rgba(2, 6, 23, 0.85);
  border: 1px solid rgba(148, 163, 184, 0.25);
}

.stats-card h3,
.help-card h3 {
  margin-top: 0;
  font-size: 0.92rem;
}

.help-card ul {
  padding-left: 18px;
  margin: 4px 0;
  font-size: 0.8rem;
  color: var(--muted);
}

.content {
  background: transparent;
}

/* Tabs */

.mode-switch {
  display: flex;
  gap: 8px;
}

.tab-button {
  border-radius: 999px;
  border: 1px solid var(--border-subtle);
  padding: 6px 14px;
  font-size: 0.85rem;
  cursor: pointer;
  background: transparent;
  color: var(--muted);
  transition: background 0.2s, color 0.2s, border 0.2s, transform 0.1s;
}

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

.tab-button.active {
  background: var(--accent-soft);
  border-color: var(--accent-strong);
  color: var(--accent-strong);
}

/* Views */

.view {
  display: none;
}

.view.active {
  display: block;
}

/* Flashcards */

.flashcard-container {
  display: flex;
  justify-content: center;
  margin-bottom: 12px;
}

.flashcard {
  width: min(700px, 100%);
  min-height: 220px;
  background: linear-gradient(135deg, rgba(15, 23, 42, 0.98), #020617);
  border-radius: 22px;
  border: 1px solid rgba(148, 163, 184, 0.4);
  box-shadow: var(--shadow-soft);
  padding: 20px 20px;
  cursor: pointer;
  position: relative;
  transition: transform 0.3s ease, box-shadow 0.3s ease;
  transform-style: preserve-3d;
}

.flashcard-front,
.flashcard-back {
  position: absolute;
  inset: 0;
  padding: 20px 20px;
  backface-visibility: hidden;
}

.flashcard-back {
  transform: rotateY(180deg);
}

.flashcard.flipped .flashcard-front {
  transform: rotateY(180deg);
}

.flashcard.flipped .flashcard-back {
  transform: rotateY(360deg);
}

.flashcard:hover {
  transform: translateY(-4px);
  box-shadow: 0 24px 55px rgba(15, 23, 42, 0.9);
}

.flashcard p {
  font-size: 0.98rem;
  line-height: 1.5;
}

.topic-tag {
  display: inline-block;
  padding: 4px 10px;
  border-radius: 999px;
  border: 1px solid rgba(148, 163, 184, 0.6);
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--muted);
  margin-bottom: 10px;
}

.flashcard-hint {
  margin-top: 14px;
  font-size: 0.8rem;
  color: var(--muted);
}

/* Flashcard progress */

.flashcard-progress {
  width: min(700px, 100%);
  margin: 0 auto 10px auto;
}

.flashcard-progress-text {
  font-size: 0.85rem;
  color: var(--muted);
  margin-bottom: 4px;
}

.progress-bar {
  width: 100%;
  height: 6px;
  border-radius: 999px;
  background: rgba(15, 23, 42, 0.9);
  border: 1px solid rgba(148, 163, 184, 0.4);
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  width: 0%;
  border-radius: 999px;
  background: linear-gradient(90deg, var(--accent-soft), var(--accent-strong));
  transition: width 0.25s ease-out;
}

.flashcard-controls {
  display: flex;
  justify-content: center;
  gap: 8px;
  flex-wrap: wrap;
}

/* Buttons */

.btn-primary,
.btn-secondary,
.btn-soft {
  border-radius: 999px;
  padding: 8px 16px;
  border: 1px solid transparent;
  font-size: 0.85rem;
  cursor: pointer;
  transition: background 0.18s, color 0.18s, transform 0.1s, border 0.18s;
}

.btn-primary {
  background: var(--accent-strong);
  color: #0b1120;
  border-color: var(--accent-strong);
}

.btn-primary:hover {
  transform: translateY(-1px);
  filter: brightness(1.05);
}

.btn-secondary {
  background: transparent;
  border-color: var(--border-subtle);
  color: var(--text);
}

.btn-secondary:hover {
  background: rgba(15, 23, 42, 0.9);
}

.btn-soft {
  background: var(--accent-soft);
  border-color: var(--accent-soft);
  color: var(--accent-strong);
}

.btn-soft:hover {
  background: rgba(56, 189, 248, 0.2);
}

/* Quiz */

.quiz-setup {
  background: rgba(15, 23, 42, 0.9);
  border-radius: var(--radius-lg);
  padding: 14px;
  border: 1px solid var(--border-subtle);
  margin-bottom: 12px;
  box-shadow: var(--shadow-soft);
}

.form-row {
  margin-bottom: 10px;
}

.form-row label {
  display: block;
  font-size: 0.85rem;
  color: var(--muted);
  margin-bottom: 4px;
}

.form-row select,
.form-row input {
  width: 100%;
  padding: 8px 10px;
  border-radius: var(--radius-md);
  border: 1px solid var(--border-subtle);
  background: #020617;
  color: var(--text);
}

.small-note {
  font-size: 0.8rem;
  color: var(--muted);
  margin-top: 4px;
}

.quiz-area {
  background: rgba(15, 23, 42, 0.9);
  border-radius: var(--radius-lg);
  padding: 14px;
  border: 1px solid var(--border-subtle);
  box-shadow: var(--shadow-soft);
  margin-bottom: 12px;
}

.quiz-meta {
  display: flex;
  justify-content: space-between;
  font-size: 0.85rem;
  color: var(--muted);
  margin-bottom: 8px;
}

.quiz-question-card {
  padding: 10px;
  border-radius: var(--radius-md);
  background: rgba(2, 6, 23, 0.9);
  border: 1px solid rgba(148, 163, 184, 0.35);
}

.quiz-options {
  margin-top: 8px;
  display: grid;
  gap: 6px;
}

.quiz-option-btn {
  text-align: left;
  padding: 8px 10px;
  border-radius: var(--radius-md);
  border: 1px solid rgba(148, 163, 184, 0.4);
  background: #020617;
  color: var(--text);
  cursor: pointer;
  font-size: 0.9rem;
  transition: background 0.15s, border 0.15s, transform 0.1s;
}

.quiz-option-btn:hover {
  transform: translateY(-0.5px);
  background: rgba(15, 23, 42, 0.9);
}

.quiz-option-btn.correct {
  border-color: var(--success);
  background: rgba(34, 197, 94, 0.12);
}

.quiz-option-btn.incorrect {
  border-color: var(--danger);
  background: rgba(248, 113, 113, 0.12);
}

.quiz-feedback {
  margin-top: 10px;
  font-size: 0.85rem;
}

.quiz-feedback.correct {
  color: var(--success);
}

.quiz-feedback.incorrect {
  color: var(--danger);
}

.quiz-controls {
  display: flex;
  justify-content: flex-end;
  margin-top: 8px;
}

.quiz-summary {
  background: rgba(15, 23, 42, 0.9);
  border-radius: var(--radius-lg);
  padding: 14px;
  border: 1px solid var(--border-subtle);
  box-shadow: var(--shadow-soft);
}

/* Footer */

.app-footer {
  text-align: center;
  padding: 8px 12px;
  font-size: 0.78rem;
  color: var(--muted);
  border-top: 1px solid rgba(15, 23, 42, 0.7);
  background: rgba(15, 23, 42, 0.95);
}

/* Responsive */

@media (max-width: 900px) {
  .app-main {
    grid-template-columns: 1fr;
    padding: 10px;
  }

  .sidebar {
    order: 2;
    margin-top: 10px;
  }

  .content {
    order: 1;
  }
}

@media (max-width: 600px) {
  .app-header {
    flex-direction: column;
    align-items: flex-start;
    gap: 6px;
  }

  .flashcard {
    min-height: 260px;
    box-shadow: 0 16px 32px rgba(15, 23, 42, 0.8);
  }

  .app-main {
    padding: 8px;
  }

  .login-card {
    padding: 18px 14px;
  }
}