Migration: Add color replacement schematics

[gfellerph]

Some of these variables can be replaced by their new equivalents automatically. Extend the schematics to handle that.

$white-alpha-x: rgba(var(--white-rgb), x/10);
$black-alpha-x: rgba(var(--black-rgb), x/10);

$primary-color: $gray-80;
$secondary-color: $gray-60;

// All renamed vars