Replace hard-coded hex with scss color variables

[mkevins]

In many places, we are using hard-coded hex values for colors, even when there are variables for those colors in src/_colors.scss. We should replace all the hard-coded values with the variables where possible.

Example of this replacement: WordPress/gutenberg@e6e7dcf

Some related issues:

Color naming conventions: WordPress/gutenberg#19352
Improving scss usage within this project: #875