.main-wrap{
  position: relative;
  z-index: 1;
}

/* 背景 */
.main-wrap::before{
  content: "";
  position: absolute;
  inset: 0;
  background: url("https://treetop-nail.com/system_panel/uploads/images/bg7.jpg") center center / cover no-repeat;
  background-attachment: fixed;
  z-index: -1;
}