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/_base.scss

@@ -118,8 +118,13 @@ a {
 
   &:hover,
   &:active {
+    color: $link-color-hover;
     outline: 0;
   }
+
+  &:visited {
+    color: $link-color-visited;
+  }
 }
 
 /* code */
@@ -141,7 +146,7 @@ td > code {
   padding-bottom: 0.1rem;
   font-size: $type-size-6;
   background: $code-background-color;
-  border: 1px solid $lighter-gray;
+  border: 1px solid $border-color;
   border-radius: $border-radius;
   box-shadow: $box-shadow;
 
@@ -236,20 +241,16 @@ figure {
 
 figcaption {
   margin-bottom: 0.5em;
-  color: mix(#fff, $text-color, 25%);
+  color: $muted-text-color;
   font-family: $caption-font-family;
   font-size: $type-size-6;
 
   a {
-    color: inherit;
-    text-decoration: none;
-    border-bottom: 1px solid $light-gray;
     -webkit-transition: $global-transition;
     transition: $global-transition;
 
     &:hover {
-      color: #000;
-      border-bottom-color: #000;
+      color: $link-color-hover;
     }
   }
 }