Peep the CodePen demo.
This PR creates a !default empty Sass maps for our $grays, $colors, and $theme-colors lists and map-merges a default setting for all colors into them. This way, anyone can continue to map-merge in additional or replacement key: value pairs as needed without having to replace the entire original Sass map as @martinbean pointed out in #23112 (closed).
This technique was pointed out by @hokiecoder in https://github.com/twbs/bootstrap/issues/22891#issuecomment-316463806.
Thoughts? If folks are interested, we'll need to update the options.md docs page to explain this, and add some code comments in our Sass files.