Add CSS variables throughout Guise Skeleton.

A central idea of Guise Skeleton going forward is easy customization based on semantic custom properties tied to Guise Skeleton styles. Some components have this somewhat already, such as --table-header-font-family and --table-stripe-background-color for a <table>. Similar CSS variables need to be added throughout.

This ticket will not initially add variables for every configurable aspect. It will just set a baseline to build on later.