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Description

The sticky table header in the datasets hit-testing page uses a transparent background in dark mode (rgb(24 24 27 / 0.6)), causing table content to visually bleed through the header when scrolling. This creates poor readability and visual confusion.

Motivation

This refactoring improves user experience by:

• Eliminating visual overlap between scrolling content and sticky headers
• Maintaining consistent UI behavior between light and dark modes
• Following UI best practices for sticky positioned elements

Additional Context

The issue affects the table at web/app/components/datasets/hit-testing/index.tsx lines 140-145. The transparent background was likely intended for aesthetic purposes but creates usability issues when combined with sticky positioning.

Light mode already uses an opaque background, so this change brings dark mode in line with that behavior.