feat(ui): add Tokyo Night theme

[danrneal]

Summary

Adds the Tokyo Night dark theme to the Gemini CLI.

Details

• Implemented the tokyonight-dark theme based on the popular Tokyo Night color palette. (https://github.com/folke/tokyonight.nvim)
• Registered the new theme in ThemeManager (packages/cli/src/ui/themes/theme-manager.ts).
• Updated the docs/cli/themes.md documentation to include the new theme in the "Dark themes" list and added a screenshot reference.
• Added the corresponding screenshot theme-tokyonight-dark.png to docs/assets/.

Related Issues

Fixes #24056

How to Validate

1. Build and run the CLI locally: npm run build && npm start
2. Open the theme selection menu using the /theme command.
3. Verify that "Tokyo Night" appears in the list of available themes.
4. Select "Tokyo Night" and ensure the CLI UI colors and syntax highlighting update correctly.
5. Verify the documentation renders correctly if viewing locally.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces the popular Tokyo Night dark theme to the Gemini CLI. The changes include the definition of the theme's color palette and syntax highlighting rules, its registration in the application's theme manager, and corresponding updates to the user documentation.

Highlights

• New Theme Implementation: Added the 'Tokyo Night' dark theme, implementing the color palette and syntax highlighting configurations.
• Theme Registration: Registered the new Tokyo Night theme within the ThemeManager to make it available in the CLI.
• Documentation Update: Updated the themes documentation to include Tokyo Night in the dark themes list and added a visual reference.

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[gemini-code-assist]

Code Review

This pull request introduces the 'Tokyo Night' dark theme to the Gemini CLI, including its color palette, syntax highlighting styles, and documentation. A review comment suggests correcting the semantic color mapping in the theme definition to ensure that color names like AccentPurple and AccentBlue correspond to their expected palette values.