Generated by SassMeister.com.

SassMeister-input-HTML.html

<button>default</button>
<button class="secondary-button">secondary</button>

SassMeister-input.scss

// ----
// Sass (v3.3.8)
// Compass (v1.0.0.alpha.19)
// ----

@function set-font-color($background-color, $default-font-color) {
    $font-color: $default-font-color;

    @if lightness($background-color) > 50% {
        $font-color: invert($default-font-color);
    }

    @return $font-color;
}

@function sfc($background-color, $default-font-color) {
    @return set-font-color($background-color, $default-font-color);
}
$white:                 #fff !default;
$gray:                  #444 !default;
$light-gray:            lighten($gray, 13.5%) !default; //#666
$lighter-gray:          lighten($gray, 33.5%) !default; //#999
$lighterer-gray:        lighten($gray, 40.5%) !default; //#ababab
$lightest-gray:   		lighten($gray, 60%) !default; //#ddd
$off-white: 			darken($white, 4.75%); //#f3f3f3
$blue:                  #002868 !default;

$button-default-bg-color: $blue !global;
$button-default-font-color: $off-white;

button {
    width: auto;
    min-width: 100px;
    height: 30px;
    text-align: center;
    font-size: 0.75em;
    line-height: 1;
    margin: 10px;
    display: inline-block;
    position: relative;
    background-color: $button-default-bg-color;
    color: sfc($button-default-bg-color, $button-default-font-color);
}

.secondary-button {
    background-color: $lighter-gray;
    color: sfc($lighter-gray, $button-default-font-color);
}

SassMeister-output.css

button {
  width: auto;
  min-width: 100px;
  height: 30px;
  text-align: center;
  font-size: 0.75em;
  line-height: 1;
  margin: 10px;
  display: inline-block;
  position: relative;
  background-color: #002868;
  color: #f3f3f3;
}

.secondary-button {
  background-color: #999999;
  color: #0c0c0c;
}

SassMeister-rendered.html

<button>default</button>
<button class="secondary-button">secondary</button>