:root {
  --bg-deep: #0f1419;
  --bg-panel: #1a2332;
  --bg-cell: #243044;
  --bg-cell-hover: #2d3d56;
  --bg-given: #1e2a3d;
  --accent: #3ee8b5;
  --accent-dim: rgba(62, 232, 181, 0.15);
  --text: #e8edf4;
  --text-muted: #8b9cb3;
  --error: #ff6b7a;
  --error-bg: rgba(255, 107, 122, 0.12);
  --border: rgba(255, 255, 255, 0.08);
  --thick: rgba(62, 232, 181, 0.35);
  --font: "DM Sans", "Segoe UI", system-ui, sans-serif;
  --radius: 14px;
  --shadow: 0 20px 50px rgba(0, 0, 0, 0.45);
}

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

html {
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
}

body {
  margin: 0;
  min-height: 100vh;
  font-family: var(--font);
  color: var(--text);
  background: var(--bg-deep);
  background-image:
    radial-gradient(ellipse 120% 80% at 50% -20%, rgba(62, 232, 181, 0.12), transparent),
    radial-gradient(ellipse 60% 40% at 100% 50%, rgba(100, 120, 255, 0.06), transparent);
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: 1.5rem 1rem 2.5rem;
}

.app-header {
  text-align: center;
  margin-bottom: 1.25rem;
}

.app-header h1 {
  margin: 0 0 0.35rem;
  font-weight: 700;
  font-size: clamp(1.75rem, 4vw, 2.25rem);
  letter-spacing: -0.02em;
  background: linear-gradient(135deg, var(--text) 0%, var(--accent) 100%);
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}

.app-header p {
  margin: 0;
  color: var(--text-muted);
  font-size: 0.95rem;
}

.toolbar {
  display: flex;
  flex-wrap: wrap;
  gap: 0.6rem;
  justify-content: center;
  align-items: center;
  margin-bottom: 1.25rem;
  max-width: 520px;
}

.toolbar-group {
  display: flex;
  flex-wrap: wrap;
  gap: 0.45rem;
  align-items: center;
}

label.difficulty-label {
  color: var(--text-muted);
  font-size: 0.8rem;
  margin-right: 0.15rem;
}

select.difficulty-select {
  appearance: none;
  background: var(--bg-panel);
  border: 1px solid var(--border);
  color: var(--text);
  padding: 0.45rem 2rem 0.45rem 0.75rem;
  border-radius: 10px;
  font-family: inherit;
  font-size: 0.875rem;
  cursor: pointer;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' fill='%238b9cb3' viewBox='0 0 16 16'%3E%3Cpath d='M8 11L3 6h10l-5 5z'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 0.6rem center;
}

select.difficulty-select:focus {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

button.btn {
  font-family: inherit;
  font-size: 0.875rem;
  font-weight: 600;
  padding: 0.5rem 1rem;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--bg-panel);
  color: var(--text);
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s, transform 0.1s;
}

button.btn:hover {
  background: var(--bg-cell-hover);
  border-color: rgba(62, 232, 181, 0.25);
}

button.btn:active {
  transform: scale(0.98);
}

button.btn-primary {
  background: linear-gradient(145deg, #2a8f72, var(--accent));
  color: #0a1210;
  border-color: transparent;
}

button.btn-primary:hover {
  filter: brightness(1.08);
  border-color: transparent;
}

.status-bar {
  min-height: 1.5rem;
  text-align: center;
  font-size: 0.9rem;
  color: var(--text-muted);
  margin-bottom: 0.75rem;
}

.status-bar.success {
  color: var(--accent);
  font-weight: 600;
}

.status-bar.error {
  color: var(--error);
}

.game-wrap {
  background: var(--bg-panel);
  border-radius: var(--radius);
  padding: clamp(0.85rem, 2vw, 1.25rem);
  box-shadow: var(--shadow);
  border: 1px solid var(--border);
}

.sudoku-grid {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  gap: 0;
  width: min(92vw, 420px);
  aspect-ratio: 1;
  background: var(--thick);
  border: 2px solid var(--thick);
  border-radius: 8px;
  overflow: hidden;
}

.cell {
  position: relative;
  background: var(--bg-cell);
  border: 1px solid var(--border);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: clamp(1rem, 3.8vw, 1.35rem);
  font-weight: 600;
  cursor: pointer;
  user-select: none;
  transition: background 0.12s;
}

/* thick box lines */
.cell:nth-child(3n):not(:nth-child(9n)) {
  border-right-width: 2px;
  border-right-color: var(--thick);
}
.cell:nth-child(9n) {
  border-right-width: 2px;
  border-right-color: var(--thick);
}
.cell:nth-child(n + 19):nth-child(-n + 27),
.cell:nth-child(n + 46):nth-child(-n + 54) {
  border-bottom-width: 2px;
  border-bottom-color: var(--thick);
}

.cell:hover {
  background: var(--bg-cell-hover);
}

.cell.selected {
  background: var(--accent-dim);
  box-shadow: inset 0 0 0 2px var(--accent);
}

.cell.given {
  background: var(--bg-given);
  color: var(--text);
  cursor: default;
}

.cell.given:hover {
  background: var(--bg-given);
}

.cell.highlight-peer {
  background: rgba(62, 232, 181, 0.08);
}

.cell.conflict {
  background: var(--error-bg);
  color: var(--error);
}

.cell.given.conflict {
  color: var(--error);
}

.numpad {
  display: grid;
  grid-template-columns: repeat(5, 1fr);
  gap: 0.45rem;
  margin-top: 1rem;
  max-width: 420px;
  margin-left: auto;
  margin-right: auto;
}

.numpad button {
  font-family: inherit;
  font-size: 1rem;
  font-weight: 600;
  padding: 0.65rem;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--bg-cell);
  color: var(--text);
  cursor: pointer;
  transition: background 0.12s;
}

.numpad button:hover {
  background: var(--bg-cell-hover);
}

.numpad button:active {
  transform: scale(0.97);
}

.numpad .clear-btn {
  grid-column: span 2;
  color: var(--text-muted);
}

.footer-hint {
  margin-top: 1.5rem;
  text-align: center;
  font-size: 0.8rem;
  color: var(--text-muted);
  max-width: 420px;
  line-height: 1.5;
}

@media (max-width: 380px) {
  .toolbar {
    flex-direction: column;
  }
}