:root {
background-image: url("https://lunarhonk.neocities.org/pets!!!/dog_bg.png");
}

.box01{
background:url(back01.gif);
box-shadow:0 1px 3px #000; /* 影 */
box-sizing:border-box;
margin:1em 0; /* 中央寄せは0をautoに変更 */
padding:20px;
width:80% /* 幅 */
}
.box-inner{
background:url(back02.gif);
box-shadow:0 1px 3px #000; /* 影 */
padding:20px
}

  .container {
  display: flex; /* or inline-flex */
flex-wrap: column wrap;
 justify-content: flex-start
  flex-direction:ltr;  
 gap: 10px;
 width: 1500px;
}