Add candidate theme from issue #406

[github-actions]

Candidate theme submission

Generated from #406.

Closes #406.

• Title: Fully Transparent Theme
• Slug: fully-transparent-theme
• Repository: https://github.com/6meowscles/firefox-transparent-toolbar
• Submitted by: @6meowscles
• Submitter role: author
• Status: candidate
• Tags: gnome, linux, minimal, transparent, wallpaper
• Screenshots: 2

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.

Notes from submitter:

This provides option for the user to make it just see the wallpaper or TRUE see-through of their desktop.

[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.