Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.3.2)
// ----

@mixin bg-color($color) {
  background-color: $color;
}

.my-thing {
  &.theme-red {
    $bg-color: red;
    
    .inner-thing {
      @include bg-color(red);
    }
  }
  
  &.theme-blue {
    @include bg-color(red);
    
    .inner-thing {
      @include bg-color(blue);
    }
  }
}

SassMeister-output.css

.my-thing.theme-red .inner-thing {
  background-color: red;
}

.my-thing.theme-blue {
  background-color: red;
}

.my-thing.theme-blue .inner-thing {
  background-color: blue;
}