## What
Adjust the highest-index chart color variables so adjacent segments stay
distinguishable:
- **Light mode:** `--chart-9`/`--chart-10` use darker zinc shades
(`zinc-700`/`zinc-600`) instead of the near-white `zinc-100`/`zinc-50`,
which were invisible against the light background.
- **Dark mode:** `--chart-7`/`--chart-8` use lighter zinc shades
(`zinc-200`/`zinc-300`) instead of the near-black `zinc-800`/`zinc-900`.
## Why
Charts with many categories were rendering low-index and high-index
segments at nearly the same value as the background, making them
unreadable.
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