docs(ui): add dark theme for storybook docs

[cylewaitforit]

🔗 Linked issue

Related: #1964

🧭 Context

This adds the dark theme to the docs pages in the npmx storybook.

Note

Because there are not any docs pages in the storybook currently, to view this check the preview of #1964 where autodocs have been turned on for button stories.
https://698b88d5157d89f1f33a6c21-ieffcbxthf.chromatic.com/?path=/docs/components-button-buttonbase--docs

📚 Description

This allows the docs to be viewed with the default npmx theme and retains the theme switcher capabilities for the story canvas portion of the docs.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
npmx.dev	Ready	Preview, Comment	Mar 13, 2026 2:15pm

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs.npmx.dev	Ignored	Preview	Mar 13, 2026 2:15pm
npmx-lunaria	Ignored		Mar 13, 2026 2:15pm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

📢 Thoughts on this report? Let us know!

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: bcf22c1e-ac8c-43c5-9a89-eac4e8279e33

📥 Commits

Reviewing files that changed from the base of the PR and between 5db5dc6 and c0362bd.

📒 Files selected for processing (4)

• .storybook/manager.ts
• .storybook/preview-head.html
• .storybook/preview.ts
• .storybook/theme.ts

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📝 Walkthrough

Walkthrough

This pull request centralises Storybook theme configuration by extracting the theme definition into a dedicated module. A new file .storybook/theme.ts is created, defining a dark theme named npmxDark with branding and styling properties. The manager configuration is updated to import and use this external theme instead of an inlined definition. The preview configuration is extended to inject the theme into the docs rendering layer. Additionally, a new preview HTML file is introduced to override canvas background styling via CSS variables.

Possibly related PRs

• feat: add storybook #1270: Modifies .storybook/preview.ts alongside this PR to configure Storybook preview settings and theme integration.

Suggested reviewers

• danielroe
• JReinhold

🚥 Pre-merge checks | ✅ 1

✅ Passed checks (1 passed)

Check name	Status	Explanation
Description check	✅ Passed	The pull request description clearly explains the purpose: adding dark theme support to Storybook docs pages whilst maintaining theme switcher functionality for story canvas.

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✨ Finishing Touches

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• Post copyable unit tests in a comment

📝 Coding Plan

• Generate coding plan for human review comments

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