Notes: Refactor CSS

What does this MR do and why?

Refactor CSS of Notes:

Removes :not(.flash-container) selectors

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

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.

Related to #424396 (closed)