Generated by SassMeister.com.

SassMeister-input-HTML.html

<a class="button-pri">Primary Button</a>
<a class="button-sec">Primary Button</a>

SassMeister-input.scss

// ----
// Sass (v3.4.9)
// Compass (v1.0.1)
// ----

$null: null;

@mixin button-style($bg: $null, $color: white) {
  background-color: $bg;
  color: $color;
  @if $bg == $null {
    box-shadow: inset 0 0 0 1px $color;
  }
}

.button-pri {
  @include button-style($bg: red);
}

.button-sec {
  @include button-style($color: red);
}

SassMeister-output.css

.button-pri {
  background-color: red;
  color: white;
}

.button-sec {
  color: red;
  box-shadow: inset 0 0 0 1px red;
}

SassMeister-rendered.html

<a class="button-pri">Primary Button</a>
<a class="button-sec">Primary Button</a>