Add candidate theme from issue #375

[github-actions]

Candidate theme submission

Generated from #375.

Closes #375.

• Title: Opera GX
• Slug: opera-gx
• Repository: https://github.com/Godiesc/firefox-gx
• Submitted by: @2034975548
• Submitter role: author
• Status: candidate
• Tags: opera gx
• 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.