Support dark mode

### Description
Eventually we may want to consider designing for dark mode. Adding as a placeholder based on our [conversation](https://github.com/getodk/web-forms/pull/364#issuecomment-2808512035) earlier in the week.

For now, Dark mode is turned off and always displays light mode.

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### Checklist
- [x] **Does it need UI/UX design?**
- If yes, describe the UI/UX requirements or attach a Figma link in the "Design" section.
- [ ] **Does it need API design?**
- If yes, specify the endpoints, methods, or data structures needed.
- [ ] **Does it need design around state flow?**
- If yes, outline the state flow requirements.
- [x] **Does it need a test plan?**
- If yes, add the test plan as a comment in this task or attach the document.
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### User Stories

- **As a** [user type],
**I want** [functionality],
**so that** [benefit].

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### Design

- **Figma Link**: [Insert URL]
- **UI/UX Design Doc Link**: [Insert URL]
- **Technical Design Doc Links**: [Insert URL]

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### Dependencies

- [dependency]
- [dependency]

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### Additional Notes

- [notes]
- [notes]

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### Related Issues

- [issue link]
- [epic issue link, if applicable]

Start by reviewing the linked conversation in pull request #364 and determine the intended dark-mode behavior and visual requirements. The issue names no files, entry points, designs, or tests, so clarify the UI/UX scope and test plan before implementation can begin. Done should mean the design is agreed and dark mode no longer always displays light mode.

Written by the indexing model from the issue text.