Google+ is being shutdown on April 2, 2019. Social sharing buttong, comment provider, and author link configs have been removed from the theme.
ref: https://support.google.com/plus/answer/9195133
* Cleanup syntax highlighting styles
* Replace hardcoded color values with [base16](http://chriskempson.com/projects/base16/) Sass variables
* Harmonize padding between GFM code blocks and `{% highlight %}` code blocks
* Improve readability of line numbers
* Add longer code block example to test double digit line numbers
* Adjust code block colors for specific skins
* 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
