:root {
  --cell: 44px;
  --thin: 1px;
  --thick: 3px;
  --given-bg: #eef3ff;
  --invalid-bg: #ffe9e9;
  --accent: #2c3e50;
  --grid: #bdbdbd;
  --grid-strong: #333333;
}

body {
  font-family: system-ui, -apple-system, Segoe UI, Roboto, Arial, sans-serif;
  background: #f7f7f9;
  color: #222;
  margin: 0;
  padding: 24px;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
}

.app {
  width: max-content;
  background: #fff;
  border-radius: 12px;
  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.08);
  padding: 16px 16px 12px;
}

h1 {
  font-size: 18px;
  margin: 0 0 12px 2px;
  color: var(--accent);
  letter-spacing: 0.2px;
}

.grid {
  display: grid;
  grid-template-columns: repeat(9, var(--cell));
  grid-template-rows: repeat(9, var(--cell));
  background: #fff;
  border: var(--thick) solid var(--grid-strong);
}

.cell {
  width: var(--cell);
  height: var(--cell);
  border: var(--thin) solid var(--grid);
  display: flex;
  align-items: center;
  justify-content: center;
}

input.sudoku {
  width: calc(var(--cell) - 2px);
  height: calc(var(--cell) - 2px);
  border: none;
  outline: none;
  text-align: center;
  font-size: 20px;
  background: transparent;
  color: #111;
  caret-color: #3b82f6;
}

input.sudoku:disabled,
input.sudoku[readonly] {
  background: var(--given-bg);
  font-weight: 600;
  color: #1f2937;
  cursor: default;
}

input.sudoku.invalid {
  background: var(--invalid-bg);
  color: #b91c1c;
}

.thick-top {
  border-top: var(--thick) solid var(--grid-strong) !important;
}
.thick-left {
  border-left: var(--thick) solid var(--grid-strong) !important;
}
.thick-right {
  border-right: var(--thick) solid var(--grid-strong) !important;
}
.thick-bottom {
  border-bottom: var(--thick) solid var(--grid-strong) !important;
}

.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-top: 12px;
}

button {
  padding: 8px 12px;
  font-size: 14px;
  border: 1px solid #cfd3da;
  border-radius: 8px;
  background: #fff;
  color: #111827;
  cursor: pointer;
  transition: background 0.15s, box-shadow 0.15s, transform 0.02s;
}
button:hover {
  background: #f3f4f6;
}
button:active {
  transform: translateY(1px);
}
button.primary {
  background: #2563eb;
  color: #fff;
  border-color: #2563eb;
}
button.primary:hover {
  background: #1d4ed8;
}

.bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  margin-top: 12px;
  color: #374151;
  min-height: 24px;
}
.msg {
  font-size: 13px;
}
.footer {
  font-size: 12px;
  color: #6b7280;
  margin-top: 8px;
  text-align: right;
}

@media (max-width: 480px) {
  :root {
    --cell: 38px;
  }
}