Support user-defined presentation themes

[lreading]

Summary

Post-v1 backlog item: support user-defined themes for presentation output.

The design needs care because themes affect rendering architecture, safety, docs, examples, and validation. A good solution may include a default theme plus user-provided CSS or structured YAML tokens.

Alternatives considered

Keep one default theme until the renderer architecture can support customization cleanly.

Affected packages

• app (presentation renderer)
• cli (@slide-spec/cli)
• docs (documentation site)
• shared (types and validation)
• Unsure