Migrate Less color variables to CSS custom properties (variables)

[anselmbradford]

Changes

• Migrate less color variables to CSS custom properties (variables)
• Update to design-system@0.42.0
• Update to cfpb-chart-builder@6.5.0

How to test this PR

1. yarn install && yarn build and the site should be unchanged.

[wpears]

Can you also take the @s out of the colorMap in js/routes/on-demand/simple-chart/common-styles.js (it doesn't matter for functionality, just consistency).

[wpears]

Probably worth just fully updating the custom styles in apps/form-explainer/css/form-explainer.less, no?

[wpears]

Awesome work on this, just two little nits.

[anselmbradford]

Probably worth just fully updating the custom styles in apps/form-explainer/css/form-explainer.less, no?

One caveat with the CSS variables is the Less utilities don't recognize them. So a utility like fade(…) wants a Less color variable to be passed in and will error if the value is a CSS variable. In the future we will be able to do this natively with relative RGB colors, but at the moment Firefox is the only modern browser that does not yet support that.

I've updated the JS and code comment names to be in CSS variable style in 2736de2