<% example do %>
  @mixin app-background($color) {
    #{if(&, '&.app-background', '.app-background')} {
      background-color: $color;
      color: rgba(#fff, 0.75);
    }
  }

  @include app-background(#036);

  .sidebar {
    @include app-background(#c6538c);
  }
  ===
  @mixin app-background($color)
    #{if(&, '&.app-background', '.app-background')}
      background-color: $color
      color: rgba(#fff, 0.75)


  @include app-background(#036)

  .sidebar
    @include app-background(#c6538c)
<% end %>