* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: Arial, Helvetica, sans-serif;
}

body {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 10px;
  padding: 20px;
  background: #f9f9f9;
}

.board {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  width: 90vmin;
  max-width: 500px;
  aspect-ratio: 1 / 1;
  border: 3px solid black;
}

.tile {
  border: 1px solid darkgray;
  width: 100%;
  height: 100%;
  text-align: center;
  font-size: calc(10px + 1.5vmin);
  outline: none;
}

.horizontal-line {
  border-bottom: 3px solid black;
}

.vertical-line {
  border-right: 3px solid black;
}

.solver-button {
  padding: 10px 25px;
  font-size: 16px;
  cursor: pointer;
  border: none;
  background: #333;
  color: white;
  border-radius: 5px;
}

.solver-button:hover {
  background: #555;
}