Customizations scroll bar is not themed

[lramos15]

Testing #333535

[vs-code-engineering]

Sharing media and context from merged PR #333634 for verification:

Before:

After:

This pull request addresses the scrollbar styling for the customizations and plugins pages to ensure they align with the dark theme colors and layout requirements. Key changes include:

• Replaced the native scrollbar with a themed overlay scrollbar (DomScrollableElement) for both the customizations and plugins pages.
• Ensured a minimum 16px right gutter to prevent content overlap with the scrollbar, while adjusting the scrollbar position to maintain a 6px gap from the content.
• Updated CSS to reflect the correct scrollbar colors using standard theme tokens.

All changes have been validated with successful client compiles, CSS lint checks, and live DOM inspections in Code OSS.

Source PR: #333634