Theme system maintenance challenges

[harperreed]

Problem

The theme system implementation is overly complex and brittle:

• Duplicated color definitions across multiple theme variants
• No central color palette management
• Theme variants (like 'ocean', 'forest', etc.) each define their own complete color sets instead of inheriting from base themes
• Custom theme extensions aren't type-safe in all usage contexts

Impact

Difficult to maintain themes, risk of inconsistencies, harder to add new themes.

Acceptance Criteria

• Create central color palette management system
• Implement theme inheritance to reduce duplication
• Add type safety for custom theme extensions
• Refactor existing themes to use new system
• Document theme creation guidelines

Priority

Medium - affects maintainability and extensibility