Implement dark/light themes

[superposition]

Description

Add theme support with dark and light modes.

Tasks

• Define CSS custom properties for colors
• Create dark theme color palette
• Create light theme color palette
• Detect system preference (prefers-color-scheme)
• Add theme toggle in header
• Persist theme preference in localStorage
• Update WebGL colors to match theme
• Style all components for both themes

Acceptance Criteria

• Both themes look polished
• System preference is respected
• Toggle switches themes smoothly

Labels

web-app, frontend, ui