Add Three New Themes: Halloween, Nebula, and Cyber Dark

[CalmhostAcct]

🎨 Overview

This pull request introduces three new fully responsive themes to VistaPanel:

1. 🎃 Halloween — Spooky orange-and-black look for the season.
2. 🌌 Nebula — Deep indigo and cyan cosmic design with animated background.
3. ⚙️ Cyber Dark — Futuristic cyberpunk theme with neon cyan glow.

All themes:

• Are based directly on the original Christmas theme (no layout breaks).
• Avoid image dependencies (use pure CSS gradients + color logic).
• Are fully mobile-friendly and Bootstrap-safe.
• Include complete theme.json metadata with color palettes.

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📁 Folder Structure