[UI/THEME]: Add Custom Color Themes & Dark Mode Selector (/theme)

[codexbt]

🚀 Goal

Add a /theme <name> slash command in Cortex CLI allowing users to switch terminal themes dynamically (monokai, dracula, catppuccin, nord, tokyo-night).

📝 Requirements

• Add /theme [name] slash command in cortex/cli.py
• Update cortex/ui/display.py theme palette dynamically
• Save selected theme preference in ~/.cortex/config.json

📁 Target File

• cortex/cli.py & cortex/ui/display.py