<===> input.scss
.theme1, .theme2 {
  .something {
    /* nothing */
  }
}

$sel: selector-nest('.theme1, .theme2', '.something');
  
#{$sel} {
  /* nothing */
}

<===> output.css
.theme1 .something, .theme2 .something {
  /* nothing */
}

.theme1 .something, .theme2 .something {
  /* nothing */
}