Colors and Custom properties

[soggybag]

Course Book is starting to have some color. We need to put this in a form that is easy to manage while work out the final color scheme and plan for future updates.

Currently all of the colors are just defined in the style sheets. Colors and other values appear in multiple places across several stylesheets. This is not maintainable for future development.

Goal move values that are shared across multiple style rules into a index.css and declare them as CSS custom properties defined under :root. We can start by moving all of the colors.

https://developer.mozilla.org/en-US/docs/Web/CSS/--*