* Made notice Sass color mixing in based on $background-color and $text-color instead of hard-coded black and white values.
* Made some style adjustments to notices to improve readability.
Notice links are slightly darkened from the notice color, mostly because the gray-on-gray default notice links were very hard to read. Rather than being $notice-color, they are `mix(#000, $notice-color, 10%)`.
The notice background mix and code-background mix can now be set with the SCSS variables $notice-background-mix and $code-notice-background-mix.
The default mix for background was adjusted to 80%, from 90%.
The default mix for code-background was adjusted to 90%, from 95%.
Skins that still didn't read well were adjusted individually.
* Adjusted sunrise $notice-background-mix to 75%
* Adjusted dark theme notice background mix colors back to the default
Co-authored-by: Tom Manner <tsmanner@us.ibm.com>
* Add link for testing purposes
* Fix excerpt link positioning in Algolia search results
* Fix Algolia search input color
* Update CHANGELOG and history
Fixes#1903
* 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
