Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.5)
// Compass (v1.0.0.alpha.18)
// ----

@mixin mytheme {
  button.cool {
    background: blue;
  }
  
  button.cooler {
    @extend button.cool;
    border: 1px solid blue;
  }
}

.mytheme {
  @include mytheme;
}

SassMeister-output.css

.mytheme button.cool, .mytheme button.cooler {
  background: blue;
}
.mytheme button.cooler {
  border: 1px solid blue;
}