body {
  background: linear-gradient(135deg, #e0eafc 0%, #cfdef3 100%);
  min-height: 100vh;
  margin: 0;
  font-family: "Segoe UI", Arial, sans-serif;
  display: flex;
  flex-direction: column;
  align-items: center;
}

.header {
  font-size: 2.5rem;
  font-weight: bold;
  margin: 40px 0 20px 0;
  color: #2d3a4b;
  letter-spacing: 2px;
  text-shadow: 0 2px 8px #b0c4de;
}

.container {
  display: flex;
  justify-content: center;
  align-items: center;
  margin-bottom: 30px;
}

.grid {
  display: flex;
  flex-direction: column;
  border: 4px solid #2d3a4b;
  border-radius: 16px;
  background: #fff;
  box-shadow: 0 8px 32px rgba(44, 62, 80, 0.15);
  padding: 8px;
}

.line {
  display: flex;
}

.numberInput {
  width: 48px;
  height: 48px;
  margin: 2px;
  background-color: #f8fafc;
  border: 2px solid #b0c4de;
  border-radius: 8px;
  font-size: 1.5rem;
  text-align: center;
  color: #2d3a4b;
  transition: background 0.2s, border 0.2s;
  outline: none;
}

.numberInput:focus {
  background-color: #e0eafc;
  border-color: #5b9bd5;
}

input[type="number"]::-webkit-inner-spin-button,
input[type="number"]::-webkit-outer-spin-button {
  -webkit-appearance: none;
  margin: 0;
}
input[type="number"] {
  -moz-appearance: textfield;
}

.line:nth-child(3),
.line:nth-child(6) {
  border-bottom: 2px solid #5b9bd5;
}

.numberInput:nth-child(3),
.numberInput:nth-child(6) {
  border-right: 2px solid #5b9bd5;
}

.buttons {
  margin: 20px 0;
  display: flex;
  justify-content: center;
}

button.solve,
button.example {
  background: linear-gradient(90deg, #5b9bd5 0%, #4a90e2 100%);
  color: #fff;
  border: none;
  border-radius: 8px;
  padding: 14px 36px;
  font-size: 1.2rem;
  font-weight: bold;
  cursor: pointer;
  box-shadow: 0 2px 8px rgba(44, 62, 80, 0.1);
  transition: background 0.2s, transform 0.2s;
  margin-right: 20px;
}

button.solve:hover,
button.example:hover {
  background: linear-gradient(90deg, #4a90e2 0%, #5b9bd5 100%);
  transform: translateY(-2px) scale(1.04);
}

.numberInput.given {
  color: #1976d2;
  background-color: #e3f0fc;
  font-weight: bold;
  border-color: #1976d2;
}

.checkbox {
  display: flex;
  align-items: center;
  margin-right: 20px;
}

#quickSolve {
  accent-color: #5b9bd5;
  width: 22px;
  height: 22px;
  margin-right: 8px;
  border-radius: 6px;
  border: 2px solid #5b9bd5;
  cursor: pointer;
  transition: box-shadow 0.2s;
}

.checkbox label {
  font-size: 1.1rem;
  color: #2d3a4b;
  font-weight: 500;
  cursor: pointer;
  user-select: none;
  transition: color 0.2s;
}

#quickSolve:checked + label {
  color: #1976d2;
  font-weight: bold;
}