visual grid color indicators

_main.scss

body{
  background: $blue;
  @include media($small-up) {
    background: purple;
  }
  @include media($medium-up) {
    background: green;
  }
  @include media($large-up) {
    background: black;
  }
  @include media($x-large-up) {
    background: gray;
  }
}