SmilesViewer molecule SVGs don't adapt to dark mode

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Problem

SmilesViewer renders molecule SVGs from OpenChemLib on a hardcoded white background (bg-white with no dark: variant). In dark mode, molecules appear as bright white rectangles against the dark page.

Suggested fix

Add a dark mode background variant to the container, and investigate whether OpenChemLib's toSVG options support configuring atom/bond colors so that molecules are legible on dark backgrounds.