V5 Reboot : Add background-color variable to hr element

Created by: devhoussam

// Variables

$hr-margin-y:                 $spacer !default;
$hr-color:                    inherit !default;
$hr-background-color:         currentColor !default;
$hr-height:                   $border-width !default;
$hr-opacity:                  .25 !default;

// Content grouping

hr {
  margin: $hr-margin-y 0;
  color: $hr-color;
  background-color: $hr-background-color;
  border: 0;
  opacity: $hr-opacity;
}

hr:not([size]) {
  height: $hr-height;
}