<===> input.scss
$color: blue;
@mixin context($class, $color: red) {
  .#{$class} {
    background-color: $color;
    @content;
    border-color: $color;
  }
}
@include context(parent) {
  @include context(child, $color: yellow) {
    color: $color;
  }
}

<===> output.css
.parent {
  background-color: red;
  border-color: red;
}
.parent .child {
  background-color: yellow;
  color: blue;
  border-color: yellow;
}