Add tests to verify that:
- Both .shop-root and .preview-frame have theme data attributes
- Theme settings changes are reflected on shop pages
- CSS file has correct selectors for both contexts
- Generated CSS cache includes all theme variables
These tests catch regressions like the selector issue where
.preview-frame[data-mood="warm"] was incorrectly transformed to
.preview-frame, .shop-root[data-mood="warm"] instead of
.preview-frame[data-mood="warm"], .shop-root[data-mood="warm"]
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>