Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.1)
// Compass (v1.0.1)
// ----

@mixin theme($bg) {
 
   
      background: $bg;
  

}

$colors: #111 #222 #333 #444 #555;
$backgrounds: #555 #444 #333 #222 #111;

@for $i from 1 through 1 {
    #theme#{$i}{
    @for $j from 1 through 5  {
  span:nth-child(#{$j}) {
    @include theme(nth($backgrounds, $j));
  }
}
}}

SassMeister-output.css

#theme1 span:nth-child(1) {
  background: #555;
}
#theme1 span:nth-child(2) {
  background: #444;
}
#theme1 span:nth-child(3) {
  background: #333;
}
#theme1 span:nth-child(4) {
  background: #222;
}
#theme1 span:nth-child(5) {
  background: #111;
}