Uses CSS light-dark() function and prefers-color-scheme media query to automatically switch between light and dark themes based on the user's browser/system preference. Key decisions: - CSS light-dark() over manual media queries (cleaner, modern syntax) - Page reload on theme change (simplest way to regenerate Plotly chart) - Plotly chart colors updated to match CSS theme Changes: - :root { color-scheme: light dark } enables browser theme support - Body, container, and stat boxes use light-dark() for colors - Plotly layout detects dark mode and sets appropriate colors - Theme change listener triggers page refresh