body {
  background: url("../img/wallpaper.png")no-repeat;
  background-size: cover;
  display: flex;
  min-height: 100vh;
  flex-direction: column;
}

main {
  flex: 1 0 auto;

}

#login {
  
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 40%;
  
}