<===> input.scss
.container {
  @for $i from 1 through 3 {
    @at-root .box-#{$i} {
      color: darken(red,($i * 5));
    }
  }

 // Control
 @at-root .outside-child {
   background-color: blue;
  }
}

<===> output.css
.box-1 {
  color: #e60000;
}
.box-2 {
  color: #cc0000;
}
.box-3 {
  color: #b30000;
}
.outside-child {
  background-color: blue;
}