feat(scrollbar): replace TODO colors with React-defined tokens

[kamja44]

Replaced placeholder colors in VerticalScrollBarView.js and constants.js with standardized React-defined colors, resolving the inline TODO comment.

Note: REACT_RESIZE_BAR_BORDER's SCROLL_BORDER is not defined in constants, so it was intentionally left unchanged.

Summary

This is one of my first contributions to open source.
I'm starting with small cleanup tasks like addressing // TODO comments to get more familiar with the codebase and contribution process.
I'm looking forward to contributing more over time 🙂

This change resolves several TODO comments related to placeholder scrollbar colors in VerticalScrollBarView.js.

Previously, the scrollbar background (REACT_RESIZE_BAR) and thumb (SCROLL_CARET) were using temporary color values that were not aligned with the React theme system. Although these values were defined in the COLORS object, they lacked corresponding CSS variables and were therefore rendered with incorrect or fallback colors.

To address this:

• Added two new color tokens to the COLORS object: SCROLL_TRACK and SCROLL_THUMB
• Updated updateColorsToMatchTheme() to read their values from --color-scroll-track and --color-scroll-thumb
• Replaced the usage of REACT_RESIZE_BAR and SCROLL_CARET in the draw() method with the new SCROLL_TRACK and SCROLL_THUMB entries

The REACT_RESIZE_BAR_BORDER color was intentionally left unchanged, as there is no corresponding CSS variable defined for it.

As a result, the scrollbar visuals are now fully integrated with the theme system. Colors are dynamically resolved from CSS variables, removing the need for hardcoded placeholder values and improving maintainability and theming consistency.

How did you test this change?

• Verified that all relevant colors are correctly populated via updateColorsToMatchTheme()
• Manually tested scrollbar rendering under both light and dark themes
• Confirmed that the following all pass:
  • yarn test
  • yarn lint
  • yarn flow dom-node
  • yarn build

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