#content {
  display: flex;
  flex-direction: row-reverse;
}
#main {
  flex: 1;
  float: left;
}
#menu {
  width: 200px;
  float: left;
}
h1 {
  text-align: center;
  color: blue;
}
.menu_item {
  font-weight: bold;
}

.gallery {
  display: grid;
  grid-template-columns: repeat(3, 1fr); /* 横3列に固定 */
  gap: 10px;
}

.gallery img {
  width: 100%;
  height: auto;
  display: block;
}