Remove direct usage of CSS variables in Minerva, and replace them with
codex design tokens again. Document this decision in the original ADR
Note: there are still a small number of CSS variables in use, including
most notably the --color-link-red fix, which broke when I removed it,
but this change takes care of all the ones that could be easily replaced
Bug: T363743
Change-Id: I7d3a9dceb908167078987de1733774c8bd4bea2f