Fresh pass at this to replace #28268. This PR is entirely aimed at providing additional variables, and not any more classes (save for perhaps light background-colors and colors in another PR). I truly wish we could generate the variables another way, but for now, this is a huge boost to customizability purely by color.
- Dynamic variables aren't possible in Sass, so this manually adds nine variations of each color.
- Adds two new functions to
shade(lighten) andtint(darken) each color. - From there, this documents the changes with docs specific classes to help communicate that these classes aren't part of Bootstrap proper.
/cc @twbs/css-review