Sadly, this doesn't work...

SASS-input.scss

// Palette

$dark-color: #000;
$light-color: #fff;

// Theme mixins

@mixin sg_themed {
  $foreground-color: $dark-color;
  $background-color: $light-color;

  @content;

  .sg_dark_theme & {
    $foreground-color: $light-color;
    $background-color: $dark-color;

    @content;
  }
}

@mixin sg_foreground_background($foreground-strength: 1, $background-strength: 1) {
  color: rgba($foreground-color, $foreground-strength);
  background-color: rgba($background-color, $background-strength);
}

@mixin sg_box_shadow($depth: 20px) {
  box-shadow: 0px 1px $depth rgba($dark-color, 0.6),
              0px 0px 0px 1px rgba($dark-color, 0.5),
              0px 0px 0px 1px rgba($foreground-color, 0.15) inset;
}

// Component

.my_component {
  @include sg_themed {
    @include sg_foreground_background(.5, .75);
    @include sg_box_shadow;
  }
}

.another_component {
  @include sg_themed {
    @include sg_foreground_background(.25, .5);
    @include sg_box_shadow(10px);
  }
}

You get Undefined variable: "$foreground-color". on line 23 at column 15, because SASS interprets from the inside out, so it's expanding the @include sg_foreground_background(.5, .75); directive before passing it as @content to the sg_themed mixin... 🐼