Document how to contribute a new theme

[Ron537]

Summary

Write a new docs/themes.md explaining the AppTheme shape and how to contribute a new theme to DPlex.

Acceptance criteria

The doc should cover:

• The AppTheme interface (UI tokens + xterm terminal palette) — point readers to src/renderer/src/services/themes.ts.
• What each token controls (e.g. bgPanel vs bgActivity vs bgInput), with a small visual diagram or reference shot of the activity-bar/sidepanel/editor regions.
• How status colors and accent variants are derived (look at applyCssVarsSync in settingsStore.ts).
• Step-by-step "add a new theme":
  1. Add an entry under THEMES in themes.ts.
  2. Pick a variant: 'light' | 'dark'.
  3. Choose accent + neutrals (link to good-contrast resources).
  4. Test in dev mode by switching themes.
• How to test contrast (mention the WCAG ratio for text/bg pairs).
• Cross-link from CONTRIBUTING.md.

Files to touch

• docs/themes.md (new)
• CONTRIBUTING.md (one-line link)

Effort: 2-3 hours, no code change required.