Resolve "Dark mode: Internal note styling extends past note container"

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR follows on from Add background to internal notes (!95176 - merged) and Removes spacing from system and discussion notes (!95824 - merged)

This MR fixes regressions when using dark mode where:

• internal note background could extended beyond the bounds of the note.
• internal note background colour changed.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable Dark Mode theme in Account > Preferences > Color Theme
2. Add an internal note to an issue / MR / incident

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #377811 (closed)