Add candidate theme from issue #357

[github-actions]

Candidate theme submission

Generated from #357.

Closes #357.

• Title: Firefox WinUI
• Slug: firefox-winui
• Repository: https://github.com/Lockframe/Firefox-WinUI
• Submitted by: @Epico1525
• Submitter role: user
• Status: candidate
• Tags: dark, fluent
• Screenshots: 1

Reviewer checklist:

• Confirm the repository is the original project or an appropriate maintained fork.
• Confirm screenshots are safe to redistribute in this catalog.
• Edit title, description, tags, and screenshot alt text if needed.
• Keep status: "candidate" until the theme is ready to appear publicly.

[github-actions]

Approved submission prepared for publishing. I switched the theme to published and assigned the next catalog index. You can merge this PR when the checks pass.