body,html {
    height: 100%;
    margin: 0px;
    padding: 0px;
}

div.LightInDark {
    background: url(https://s6.uupload.ir/files/not_mini_2_jq91.jpg) no-repeat center center fixed; 
    background-size: cover;
    height: 100%;
    width: 100%;
  }

  @media screen and (max-width:700px){
    div.LightInDark {
        background: url(https://s6.uupload.ir/files/mini_2_2xau.jpg) no-repeat center center fixed; 
        background-size: cover;
        height: 100%;
        width: 100%;
      }
  }