merge modifications

fix light theme

scss: block unused for comments

sass: fix sidebar top

sass: remove scrollbar webkit-ism

fix masthead background

enable the light side

Really stick the head

sticky head

main: cleanup

main: fix horiz scrollbar, tweak color

main: use custom scrollbar

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 

Move SCSS partials to `/_sass/minimal-mistakes` for easier CSS customization

Merge branch 'release/4.3.1' into develop

Merge branch 'release/4.3.1'

Bump version to 4.3.1

Merge branch 'release/4.3.0' into develop

Merge branch 'release/4.3.0'

Bump version to 4.3.0