A way to add an border/outline to an element without affecting its final dimensions

inside-border.scss

// default values:
$ibThickness: 2px !default;
$ibColor: black !default;
$ibAlpha: .1 !default;

// use a "variable argument" to accept any number of values.
@mixin ib($values...) {
  $borderThickness: $ibThickness;
  $borderColor: $ibColor;
  $borderAlpha: $ibAlpha;
  @each $value in $values {
    @if type_of($value) == number {
      @if unit($value) == "" {
        $borderAlpha: $value;
      }
      @else {
        // no need to limit this length to just "px"
        $borderThickness: $value;
      }
    }
    @else if type_of($value) == color {
      $borderColor: $value;
    }
  }
  box-shadow: inset 0 0 0 $borderThickness rgba($borderColor, $borderAlpha);
}