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ThemeContext adds a .theme-transition class to <html> for ~300ms during a real toggle (skipped on initial mount); a scoped base.css rule applies a uniform 0.3s color/bg/border/fill/stroke/shadow transition to the whole tree only while that class is present — so the entire UI fades together on switch, while normal interactions keep their own transitions. Respects prefers-reduced-motion. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>