body {
  margin: 0;
  padding: 0;
  font-family: sans-serif;
  background: #f0f0f0;
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
  min-height: 100vh;
}
.container {
  text-align: center;
}
#logo {
  width: 130px; /* 少し大きめに */
  margin-bottom: 10px;
}
#howto {
  text-align: left;
  margin: 10px auto;
  max-width: 300px;
}
#controls {
  margin: 10px;
}
#board {
  display: grid;
  margin: 20px auto;
}
.cell {
  width: 30px;
  height: 30px;
  border: 1px solid #999;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  user-select: none;
  background: #bbb;
}
.cell.open {
  background: #e0e0e0;
  cursor: default;
}
.cell.flag {
  background: #f88;
}