:root {
  --bg: #1f2430;
  --panel: #2b313f;
  --grid-line: #0f131b;
  --cell-bg: #f4f7fb;
  --cell-bg-alt: #e7edf5;
  --given: #1f2430;
  --solved: #2f7dd1;
  --accent: #36c08b;
  --accent-dark: #2aa074;
  --text: #e7edf5;
  --error: #e0566b;
}

* {
  box-sizing: border-box;
  margin: 0;
}

body {
  background-color: var(--bg);
  color: var(--text);
  font-family: -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  min-height: 100vh;
  display: flex;
  justify-content: center;
  padding: 24px 16px;
}

.app {
  width: 100%;
  max-width: 460px;
  text-align: center;
}

h1 {
  font-size: 1.9rem;
  letter-spacing: 0.5px;
}

.subtitle {
  margin: 6px 0 20px;
  color: #9aa6b8;
  font-size: 0.95rem;
}

.board {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  width: 100%;
  aspect-ratio: 1 / 1;
  margin: 0 auto;
  background-color: var(--grid-line);
  border: 3px solid var(--grid-line);
  border-radius: 8px;
  overflow: hidden;
}

.cell {
  position: relative;
  border: 1px solid #c4cedd;
}

/* Thicker lines on 3x3 box boundaries. */
.cell.box-left   { border-left-width: 2px;  border-left-color: var(--grid-line); }
.cell.box-right  { border-right-width: 2px; border-right-color: var(--grid-line); }
.cell.box-top    { border-top-width: 2px;   border-top-color: var(--grid-line); }
.cell.box-bottom { border-bottom-width: 2px; border-bottom-color: var(--grid-line); }

.cell input {
  width: 100%;
  height: 100%;
  border: none;
  outline: none;
  text-align: center;
  background-color: var(--cell-bg);
  color: var(--given);
  font-size: clamp(1.1rem, 6vw, 1.7rem);
  font-weight: 600;
  caret-color: var(--accent-dark);
}

.cell input:focus {
  background-color: #d4e6fb;
}

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

.cell input.solved {
  color: var(--solved);
}

.status {
  min-height: 1.4rem;
  margin: 16px 0;
  font-weight: 600;
}

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

.controls {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
}

.field {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 0.95rem;
}

select {
  font-size: 1rem;
  padding: 6px 10px;
  border-radius: 8px;
  border: 1px solid #4a5266;
  background-color: var(--panel);
  color: var(--text);
  cursor: pointer;
}

.buttons {
  display: flex;
  gap: 12px;
}

button {
  font-size: 1rem;
  font-weight: 600;
  padding: 10px 24px;
  border: none;
  border-radius: 24px;
  background-color: var(--accent);
  color: #11331f;
  cursor: pointer;
  transition: transform 0.15s ease, background-color 0.15s ease;
}

button:hover { background-color: var(--accent-dark); transform: translateY(-1px); }
button:active { transform: translateY(0); }

button.secondary {
  background-color: transparent;
  color: var(--text);
  border: 1px solid #4a5266;
}

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