feat(cli): add configurable syntax highlighting themes via code-theme setting

[devin-ai-integration]

Description

Refs fern-api/fern-platform#8209

Adds a new code-theme option to docs.yml settings that lets customers override the Shiki syntax highlighting themes used in code blocks, with separate light/dark mode support.

Link to Devin Session: https://app.devin.ai/sessions/1012460edf4844c08456fc8e5a17f727
Requested by: @willkendall01

Changes Made

• Added CodeThemeConfig type to Fern API definition (fern/apis/docs-yml/definition/docs.yml) with optional light and dark string fields
• Added code-theme field to DocsSettingsConfig in the API definition
• Created SDK API and serialization type files for CodeThemeConfig
• Updated DocsSettingsConfig in both API and serialization layers to include codeTheme
• Updated Zod schemas (DocsYmlSchemas.ts, DocsSettingsConfigSchema.ts) with the new field
• Regenerated JSON schemas (docs-yml.schema.json at root and packages/cli/workspace/loader/src/) via pnpm jsonschema
• Added CLI changelog entry in versions.yml (v4.18.0)

Example docs.yml usage:

settings:
  code-theme:
    light: github-light
    dark: dracula

Updates Since Last Revision

• Removed codeTheme plumbing from parseDocsConfiguration.ts: The return type (CjsFdrSdk.docs.v1.commons.DocsSettingsConfig) comes from the published FDR CJS SDK on npm, which doesn't include codeTheme yet. The field will be plumbed through once fern-platform#8209 merges, the FDR SDK is regenerated/published, and this repo consumes the updated package. For now, the CLI accepts and validates the config but does not forward it at runtime.
• Regenerated JSON schema files: Previously hand-edited, now regenerated via pnpm jsonschema to match the correct definition ordering and formatting expected by CI lint.
• Added versions.yml entry: v4.18.0 feat entry for the new code-theme setting.

Companion PR

This PR provides the schema/CLI side. The runtime consumption (applying themes to Shiki) is in fern-api/fern-platform#8209. Both PRs are needed for the feature to work end-to-end.

Deployment order: fern-platform#8209 should merge first → FDR SDK gets republished → then a follow-up commit here adds the parseDocsConfiguration.ts plumbing to forward codeTheme to the platform.

Review Checklist

• Verify the hand-written SDK types (CodeThemeConfig.ts in api/serialization) match what Fern would auto-generate from the API definition — they carry the "auto-generated" comment
• Confirm "code-theme" serialization key in DocsSettingsConfig serializer correctly maps to the kebab-case API field
• Verify v4.18.0 is the correct next version in versions.yml
• Note: parseDocsConfiguration.ts does not forward codeTheme yet — this is intentional (FDR SDK dependency, see above). Confirm this gap is acceptable.
• Note: theme names are accepted as free-form strings with no validation against Shiki's BundledTheme list — invalid names will fall back to defaults at runtime (handled in the platform PR)

Testing

• pnpm run check (Biome) passes
• pnpm compile passes (including @fern-api/configuration-loader)
• pnpm jsonschema regenerated and committed
• All required CI checks pass (compile, lint, biome, test, test-ete, depcheck, versions.yml validation)
• Manual end-to-end testing requires both this PR and fern-platform#8209 to be deployed

[devin-ai-integration]

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