docs: custom themes in extensions

[jackwotherspoon]

Summary

Add docs missing for extensions bringing their own themes.

Follow up to #17327 which added the feature.

[gemini-code-assist]

Summary of Changes

Hello @jackwotherspoon, 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 enhances the documentation for Gemini CLI by thoroughly covering the functionality of custom themes provided through extensions. It clarifies how extensions can define and expose themes, how users can select them, and updates existing examples to ensure consistency and accuracy. The changes aim to provide comprehensive guidance for developers looking to integrate custom themes into their Gemini CLI extensions.

Highlights

• Documentation for Extension Themes: New documentation has been added explaining how extensions can provide custom themes for the Gemini CLI, including how they are identified and selected.
• Extension Capabilities Update: The core documentation for Gemini CLI extensions has been updated to explicitly list 'themes' as one of the capabilities that extensions can package and provide.
• Configuration Reference: The extension reference documentation now includes details on the themes property within gemini-extension.json, complete with a JSON example of a custom theme definition.
• Example Theme Consistency: An existing example extension's README and gemini-extension.json have been updated to reflect the correct theme naming convention and settings file format (JSON).

Changelog

• docs/cli/themes.md
  • Added a new section 'Themes from extensions' explaining how extensions can provide custom themes and how they are displayed.
• docs/extensions/index.md
  • Included 'themes' in the introductory list of capabilities that Gemini CLI extensions can package.
• docs/extensions/reference.md
  • Documented the themes property within the gemini-extension.json structure.
  • Added a new 'Themes' section with a detailed JSON example for defining custom themes in extensions.
  • Provided instructions on how to select extension-provided themes via the /theme command or settings.json.
• docs/extensions/writing-extensions.md
  • Integrated 'Custom themes' into the table outlining various ways to customize Gemini CLI using extensions.
• packages/cli/src/commands/extensions/examples/themes-example/README.md
  • Updated the example settings file format from config.yaml to settings.json.
  • Corrected the example theme name from shades-of-green-theme to shades-of-green.
• packages/cli/src/commands/extensions/examples/themes-example/gemini-extension.json
  • Renamed the example theme from shades-of-green-theme to shades-of-green to match documentation.

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-cli]

Hi @jackwotherspoon, thank you so much for your contribution to Gemini CLI! We really appreciate the time and effort you've put into this.

We're making some updates to our contribution process to improve how we track and review changes. Please take a moment to review our recent discussion post: Improving Our Contribution Process & Introducing New Guidelines.

Key Update: Starting January 26, 2026, the Gemini CLI project will require all pull requests to be associated with an existing issue. Any pull requests not linked to an issue by that date will be automatically closed.

Thank you for your understanding and for being a part of our community!

[gemini-code-assist]

Code Review

This pull request adds documentation for using custom themes within extensions. The changes are mostly in markdown files and look good, clarifying how to package and use themes in extensions. I found one point of confusion in the documentation where a link points to a guide with a legacy theme format, while the example shows a new format. I've added a suggestion to clarify this and avoid user confusion. Otherwise, the updates are consistent and well-written.

[github-actions]

Size Change: -2 B (0%)

Total Size: 24.4 MB

ℹ️ View Unchanged

Filename	Size	Change
./bundle/gemini.js	24.4 MB	-2 B (0%)
./bundle/sandbox-macos-permissive-open.sb	890 B	0 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB	0 B
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB	0 B
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB	0 B
./bundle/sandbox-macos-strict-open.sb	4.82 kB	0 B
./bundle/sandbox-macos-strict-proxied.sb	5.02 kB	0 B

_{compressed-size-action}

[jacob314]