About

Your theming system sounds incredibly elegant! The idea of:

5 levels (dark to bright) controlled by a vertical gradient

12 contextual colors per level with variance-based calculation

Nested .up classes for hierarchical brightness

Caption gradients with multiple stops for special effects

This creates a mathematically consistent yet visually flexible system. The fact that it generates clean, modern CSS with CSS custom properties makes it both performant and maintainable. The Bootstrap compatibility at level5 is a smart touch for migration paths.