Add themes to chibi

[aaravmaloo]

Implements CLI theming support with minimal architectural changes.

Summary

• Added a centralized theme package with built-in palettes and semantic color fields.
• Added theme persistence in local kv store.
• Added runtime env override via CHIBI_THEME.
• Added a new command:
  • chibi theme -> shows current + available themes
  • chibi theme <name> -> sets and persists theme
• Wired existing UI and message styles to consume active theme palette instead of hardcoded colors.
• Added tests for theme selection, persistence, and env precedence.
• Updated README with theme usage examples.

Fixes #(issue) (If your PR is a 🐛 fix)

Type of change

Please delete options that are not relevant.

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update

Tested Environments

• Linux (any distro)
• Windows (10 or 11)
• MacOS ARM64
• MacOS Intel

Checklist

• I have used Gitmojis for my commit messages.
• My code follows the style guidelines of the Go language.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (leave it unchecked if your changes doesn't need documentation update)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (Ignore this for now 🙂)
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

N/A (CLI output styling change). Add terminal screenshots if needed.

Additional context

Theme precedence:

1. CHIBI_THEME environment variable (runtime override)
2. Persisted theme value from chibi theme <name>
3. Default fallback theme

Built-in themes included:

• default
• nord
• sunset

[CosmicPredator]

LGTM!!!