Flint ThemeSpec authoring

Create a reusable visual system for Flint charts. Translate the user's reference materials into one valid ThemeSpec that can be pasted directly into Theme Lab.

Authoritative references

Read these before authoring. When prose and source disagree, the TypeScript contract is authoritative.

• Theme guide: https://github.com/microsoft/flint-chart/blob/main/docs/theme-spec.md
• Exhaustive ThemeSpec types and allowed values: https://github.com/microsoft/flint-chart/blob/main/packages/flint-js/src/core/theme/types.ts
• Real preset implementations: https://github.com/microsoft/flint-chart/tree/main/packages/flint-js/src/core/theme/presets
• Theme Lab: https://microsoft.github.io/flint-chart/#/theme-lab

Do not invent keys. If a requested decision cannot be represented by ThemeSpec, omit it and briefly identify the unsupported decision outside the JSON only when the user asked for an explanation.