:root {
  --bg-gradient: linear-gradient(180deg, #020617 0%, #0f172a 100%);
  --panel-bg: rgba(15, 23, 42, 0.82);
  --panel-border: rgba(56, 189, 248, 0.25);
  --text-color: #f8fafc;
  --muted-color: #94a3b8;
  --accent-color: #38bdf8;
  --accent-strong: #f97316;
  --grid-line: rgba(148, 163, 184, 0.12);
  --board-shadow: 0 24px 40px rgba(2, 6, 23, 0.55);
  --tile-shadow: rgba(15, 23, 42, 0.4);
}

[data-theme="light"] {
  --bg-gradient: linear-gradient(180deg, #f8fafc 0%, #dbeafe 100%);
  --panel-bg: rgba(241, 245, 249, 0.92);
  --panel-border: rgba(148, 163, 184, 0.25);
  --text-color: #0f172a;
  --muted-color: #475569;
  --accent-color: #2563eb;
  --accent-strong: #f97316;
  --grid-line: rgba(148, 163, 184, 0.18);
  --board-shadow: 0 22px 36px rgba(15, 23, 42, 0.18);
  --tile-shadow: rgba(148, 163, 184, 0.4);
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  background: var(--bg-gradient);
  color: var(--text-color);
  font-family: 'Inter', 'Press Start 2P', ui-sans-serif, system-ui;
  line-height: 1.6;
}

.container {
  max-width: 1100px;
  margin: 0 auto;
  padding: 32px 20px 48px;
  display: flex;
  flex-direction: column;
  gap: 28px;
}

.header {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  align-items: center;
  gap: 16px;
}

.title {
  margin: 0;
  font-size: clamp(2.2rem, 4.5vw, 3.4rem);
  letter-spacing: 0.08em;
  text-transform: uppercase;
  background: linear-gradient(120deg, var(--accent-color), var(--accent-strong));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  text-shadow: 0 0 24px rgba(59, 130, 246, 0.32);
}

.subtitle {
  margin: 4px 0 0;
  color: var(--muted-color);
  font-size: 1rem;
}

.header-actions {
  display: flex;
  gap: 12px;
}

.btn {
  border: none;
  border-radius: 14px;
  padding: 12px 18px;
  font-size: 0.9rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  cursor: pointer;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.btn.primary {
  background: linear-gradient(130deg, var(--accent-color), var(--accent-strong));
  color: #0f172a;
  box-shadow: 0 14px 26px rgba(59, 130, 246, 0.3);
}

.btn.secondary {
  background: rgba(15, 23, 42, 0.6);
  color: var(--text-color);
  border: 1px solid var(--panel-border);
}

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

.btn.secondary[hidden] {
  display: none;
}

.layout {
  display: grid;
  grid-template-columns: minmax(320px, 1fr) 320px;
  gap: 24px;
  align-items: start;
}

.board-section {
  display: flex;
  flex-direction: column;
  gap: 18px;
}

.board-wrapper {
  position: relative;
  border-radius: 22px;
  padding: 18px;
  border: 1px solid var(--panel-border);
  background: rgba(15, 23, 42, 0.65);
  box-shadow: var(--board-shadow);
  aspect-ratio: 1 / 1;
}

.back-grid {
  position: absolute;
  top: 18px;
  left: 18px;
  right: 18px;
  bottom: 18px;
  border-radius: 18px;
  overflow: hidden;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-template-rows: repeat(4, 1fr);
  gap: 6px;
}

.grid-row {
  display: contents;
}

.grid-cell {
  background: rgba(15, 23, 42, 0.35);
  border-radius: 12px;
}

.tile-container {
  position: absolute;
  top: 18px;
  left: 18px;
  width: calc(100% - 36px);
  height: calc(100% - 36px);
  pointer-events: none;
}

.tile {
  position: absolute;
  width: calc(25% - 6px);
  height: calc(25% - 6px);
  border-radius: 12px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
  font-size: 2rem;
  color: #0f172a;
  box-shadow: 0 12px 24px var(--tile-shadow);
  transition: top 0.16s ease, left 0.16s ease, transform 0.15s ease, background 0.2s ease;
}

.tile.tile-new {
  animation: pop 200ms ease;
}

.tile.tile-merged {
  animation: pulse 220ms ease;
}

.tile-container.reduced-motion .tile {
  transition: none !important;
  animation: none !important;
}

@keyframes pop {
  0% { transform: scale(0.4); opacity: 0; }
  100% { transform: scale(1); opacity: 1; }
}

@keyframes pulse {
  0% { transform: scale(1.1); }
  100% { transform: scale(1); }
}

.game-message {
  position: absolute;
  inset: 18px;
  border-radius: 18px;
  background: rgba(15, 23, 42, 0.82);
  border: 1px solid var(--panel-border);
  display: none;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  gap: 16px;
  text-align: center;
  color: var(--text-color);
  z-index: 5;
  backdrop-filter: blur(12px);
}

.game-message.show {
  display: flex;
}

.game-message p {
  font-size: 1.4rem;
  font-weight: 700;
  margin: 0;
}

.message-actions {
  display: flex;
  gap: 12px;
}

.score-pop {
  position: absolute;
  top: 14px;
  right: 24px;
  font-weight: 600;
  color: var(--accent-color);
  opacity: 0;
  transform: translateY(0);
  transition: opacity 0.4s ease, transform 0.4s ease;
}

.score-pop.show {
  opacity: 1;
  transform: translateY(-12px);
}

.sidebar {
  display: flex;
  flex-direction: column;
  gap: 18px;
}

.card {
  background: var(--panel-bg);
  border: 1px solid var(--panel-border);
  border-radius: 18px;
  padding: 20px;
  box-shadow: var(--board-shadow);
  backdrop-filter: blur(14px);
}

.card h2 {
  margin: 0 0 12px;
  font-size: 0.95rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted-color);
}

.card.stats {
  display: grid;
  gap: 14px;
}

.stat {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-size: 0.95rem;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: var(--muted-color);
}

.stat strong {
  color: var(--accent-color);
  font-size: 1.2rem;
}

.card.info p,
.card.tips li {
  color: var(--muted-color);
  font-size: 0.92rem;
}

.card.tips ul {
  margin: 0;
  padding-left: 18px;
  display: grid;
  gap: 6px;
}

.mobile-controls {
  display: grid;
  gap: 12px;
  border-radius: 16px;
  border: 1px solid var(--panel-border);
  background: rgba(15, 23, 42, 0.7);
  padding: 16px;
}

.mobile-instructions {
  text-align: center;
  font-size: 0.85rem;
  color: var(--muted-color);
}

.control-pad {
  display: grid;
  grid-template-columns: repeat(3, 60px);
  justify-content: center;
  gap: 8px;
}

.control-btn {
  border: 1px solid var(--panel-border);
  border-radius: 12px;
  background: rgba(15, 23, 42, 0.8);
  color: var(--text-color);
  font-size: 1.2rem;
  padding: 10px;
  cursor: pointer;
  transition: transform 0.15s ease;
}

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

.theme-toggle {
  position: fixed;
  right: 18px;
  bottom: 18px;
  border-radius: 999px;
  border: 1px solid var(--panel-border);
  background: var(--panel-bg);
  color: var(--text-color);
  padding: 10px 12px;
  box-shadow: var(--board-shadow);
  cursor: pointer;
}

@media (max-width: 960px) {
  .layout {
    grid-template-columns: 1fr;
  }
  .sidebar {
    order: 1;
  }
  .board-section {
    order: 0;
  }
}

@media (max-width: 600px) {
  .container {
    padding: 24px 16px 32px;
  }
  .header-actions {
    width: 100%;
    justify-content: space-between;
  }
  .control-pad {
    grid-template-columns: repeat(3, 1fr);
  }
}