Fix docs on theme to reflect the removal of --theme

[panglesd]

Use frontmatter in the examples.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Reviewing files that changed from the base of the PR and between d9f4176 and 1a5eb09.

📒 Files selected for processing (1)

• docs/themes.rst

✅ Files skipped from review due to trivial changes (1)

• docs/themes.rst

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📝 Walkthrough

Walkthrough

This PR updates the documentation in docs/themes.rst to reflect theme selection via presentation frontmatter instead of command-line arguments. The "Using a theme" section now includes examples showing how to specify a builtin theme name, a local CSS file path, or a CSS URL in the theme: frontmatter field. The rewrite removes prior examples of --theme CLI usage and --css extension references, while integrating slipshow themes list command output into the documentation narrative.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: updating documentation to reflect the removal of the --theme command-line argument.
Description check	✅ Passed	The description is brief but directly related to the changeset, confirming the use of frontmatter in examples to update documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/themes.rst`:
- Around line 11-17: Change the heading sentence to fix grammar and make the
example consistent with the described theme: replace "Here is an example of a
frontmatter to using the "vanier" predefined theme:" with "Here is an example of
frontmatter using the \"vanier\" predefined theme:" and update the example
frontmatter value from `theme: default` to `theme: vanier` so the prose and the
code block match.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: df0ea3e9-ae49-494f-a8ab-79e9defe4f15

📥 Commits

Reviewing files that changed from the base of the PR and between 3b0274d and d9f4176.

📒 Files selected for processing (1)

• docs/themes.rst