Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@mixin for-theme($theme) {
    @media (theme: $theme) {
      & {
        @content;
      }
    }
}

.Menu {
  position: fixed; // or something
  
  @include for-theme('default') {
    color: #333;
  }
  @include for-theme('tuintuin') {
    color: green;
  }
}

SassMeister-output.css

.Menu {
  position: fixed;
}

@media (theme: default) {
  .Menu {
    color: #333;
  }
}

@media (theme: tuintuin) {
  .Menu {
    color: green;
  }
}