result.css

.container {
  background: #fff;
  color: #333;
}
.container.dark {
  background: #333;
  color: #fff;
}

theme-scoping.less

#theme( @theme ) when ( @theme = 'dark' ){
  .color(){
    color: #fff;
  }

  .background(){
    background: #333;
  }
}

#theme( @theme ) when ( @theme = 'light' ){
  .color(){
    color: #333;
  }

  .background(){
    background: #fff;
  }
}

.container {
  #theme('light');
    .background();
    .color();

  &.dark {
    #theme('dark');
      .background();
      .color();
  }
}