Overhaul navigation styles to light theme
Fix navigation dropdown borders
Fix navigation dropdown positioning
Fix navigation responsive styles
Refactor form toolbar to new "addons" partial folder

Helpers:
Add `content-box` helper

Typography:
Add code, pre, and kbd styles for missing normalize styles

Demo:
Update demo styles and content
Remove cc IE comments in favour or feature detection
Fix demo icons

Index:
Remove navigation footer
Fix navigation links