Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.9)
// Compass (v1.0.0.alpha.20)
// ----

$themeName: Z;

@mixin theme($themeName) {
  @if $themeName == z {
    @content;
  }

  @if $themeName == y {
    @content;
  }
}

.wrapper {
  @include theme(z) {
    border-top: 10px solid green;
  }  
  @include theme(y) {
    border-top: 10px solid blue;
  }
}

SassMeister-output.css

.wrapper {
  border-top: 10px solid green;
  border-top: 10px solid blue;
}