Introduces CSS custom properties for the purposes
of enabling client-side customizable font sizes.
CSS custom properties are contained in a Less file,
which imports values from Codex and converted them to rems
using the Less unit() function.
Includes ADR on CSS custom properties decisions.
Bug: T348984
Change-Id: Ic5e62d55f3c3585fe3a00e41f29f88d3a2f7470b
