Color "skins" (#1242)

* Add YIQ Color Contrast mixin
* Remove text underline from `.btn` links
* Move YIQ variables
* Simplify button classes using YIQ color contrast mixin
  - DRY up button CSS using Sass list and YIQ color contrast mixin.
  - Move `color` and `background-color` to new `btn--primary` class instead of assinging on the default class. Removes the need to override them.
* Add `.btn--primary` to buttons that just had `.btn`
* Apply changes to `/docs`
* Add `.btn--primary` class
* Update CHANGELOG and history
* Add sample form
* Abstract colors away into Sass variables for easier themeing
* Add "dark" skin
* Replace hardcoded color with SCSS variable
* Invert Font Awesome icons' colors in author sidebar and footer
* Add Sass changes to `/docs`
* Use primary button type instead of inverse
* Add missing `!default` on `$muted-text-color`
* Add `contrast` and `sunrise` skin colors
* Add `dirt` skin color
* Add `air` skin color option
* Add `mint` skin color
* Add `btn--primary` class to Submit Comment button
* Set skin to `default`
* Document skin color options
* Add note about skin SCSS import

Close #1208

docs/_sass/minimal-mistakes/_mixins.scss

@@ -50,4 +50,43 @@
     content: "";
     display: table;
   }
+}
+
+/*
+   Compass YIQ Color Contrast
+   https://github.com/easy-designs/yiq-color-contrast
+   ========================================================================== */
+
+@function yiq-is-light(
+  $color,
+  $threshold: $yiq-contrasted-threshold
+) {
+  $red: red($color);
+  $green: green($color);
+  $blue: blue($color);
+
+  $yiq: (($red*299)+($green*587)+($blue*114))/1000;
+
+  @if $yiq-debug { @debug $yiq, $threshold; }
+
+  @return if($yiq >= $threshold, true, false);
+}
+
+@function yiq-contrast-color(
+  $color,
+  $dark: $yiq-contrasted-dark-default,
+  $light: $yiq-contrasted-light-default,
+  $threshold: $yiq-contrasted-threshold
+) {
+  @return if(yiq-is-light($color, $threshold), $yiq-contrasted-dark-default, $yiq-contrasted-light-default);
+}
+
+@mixin yiq-contrasted(
+  $background-color,
+  $dark: $yiq-contrasted-dark-default,
+  $light: $yiq-contrasted-light-default,
+  $threshold: $yiq-contrasted-threshold
+) {
+  background-color: $background-color;
+  color: yiq-contrast-color($background-color, $dark, $light, $threshold);
 }