feat: Custom theme editor

[ikorich]

Summary

Create and edit custom syntax color schemes.

Motivation

Power users want complete control over colors.

Implementation

• Color pickers for each syntax element
• Preview while editing
• Save custom themes
• Import/export themes

Syntax Elements

• Text, Keywords, Types
• Variables, Strings, Numbers
• Comments, Background, Selection

Acceptance Criteria

• Theme editor UI
• Real-time preview
• Save custom themes
• Apply custom themes

Story Points

8 points