Theming mixin for Sass; forked from http://www.developwithpurpose.com/creating-a-sass-theme-engine/

SassMeister-input.scss

// ----
// Sass (v3.4.13)
// Compass (v1.0.3)
// ----


// theme mixin

@mixin theme(
  // DEFAULTS
  $post_title: null, 
  $post_subtitle: null
) {
  
  // IF first arg is a map, pass that
  @if type-of($post_title) == 'map' { @include theme($post_title...); }
  
  // otherwise assume named arguments
  @else {
    @if $post_title {
      %theme_post_title {
        color: $post_title;
      }
    }
    @if $post_subtitle {
      %theme_post_subtitle {
        color: $post_subtitle;
      }
    }
  }
}

.post_title {
  @extend %theme_post_title !optional;
  color: black;
}

.post_subtitle {
  @extend %theme_post_subtitle !optional;
  color: black;
}

.page--forest {
  @include theme((
    post_title: forestgreen,
    post_subtitle: green
  ));
}

.page--ocean {
  @include theme((
    post_subtitle: navy
  ));
}

SassMeister-output.css

.post_title {
  color: black;
}

.post_subtitle {
  color: black;
}

.page--forest .post_title {
  color: forestgreen;
}
.page--forest .post_subtitle {
  color: green;
}

.page--ocean .post_subtitle {
  color: navy;
}