Visual polish and edge case handling (litmus-2xrw)
Visual polish, edge case handling for very bright/dark themes and low contrast scenarios.
Summary of Changes
Visual polish for edge cases:
- Theme cards show light/dark variant label and fg/bg contrast ratio
- Neutral gray borders (rgba) visible on both light and dark theme cards/scene blocks
- Box shadow for card depth regardless of background
- Relative luminance detection for light vs dark theme classification