body {
  overflow-x: hidden;
}
body:before {
  width: 50%;
  height: 100%;
  content: "";
  position: fixed;
  top: 0;
  left: 0;
  z-index: -1;
  background: #F5FDFF; /* 左側の背景 */
}

@media screen and (max-width: 767px) {
  body:before {
    display: none;
  }
}