Plan SyncDeck host theme contract

[mrbdahlem]

Summary

This PR adds a planning document for a SyncDeck host theme contract so embedded and instructor-launched activities can inherit deck colors and typography instead of falling back to the host's default white UI.

Proposed Direction

• define a normalized host theme token contract for brand, surface, text, typography, radius, and shadow
• start with a per-deck global theme passed once during presentation startup
• support optional per-activity ui.theme overrides on top of the deck theme
• keep host-owned fallback tokens so existing decks continue to render safely
• have the host map resolved theme tokens into CSS variables instead of hard-coded styling

Recommended Merge Order

host fallback theme
  -> deck theme
  -> per-activity override theme

Why This Shape

• presentation authors keep ownership of course/deck styling
• the host keeps ownership of rendering and CSS application
• payloads stay smaller when multiple activities share the same deck theme
• per-activity overrides remain available as an escape hatch instead of the default transport

Planning Scope

The plan covers:

• token families and fallback behavior
• deck startup/bootstrap integration
• payload validation and sanitization
• candidate host integration points
• compatibility expectations for existing decks
• implementation checklist and open questions

Validation

Docs-only change.

• verified the new plan file content on branch plan/syncdeck-host-theme-contract

Notes

If implementation changes SyncDeck launch payload formats, we should also update:

• skills/syncdeck/references/ACTIVITY_PAYLOADS.md
• .agent/knowledge/data-contracts.md