Update styling for SSE markdown editor

What does this MR do?

It ensures the Markdown editor encapsulated within the Static Site Editor's Toast UI Editor aligns more closely with how the rest of GitLab's editors display markdown. If Toast UI Editor used the Ace Editor under the hood (like the IDE and the single file editor) we would have more reusability. However, Toast UI uses Code Mirror under the hood. We are able to reuse SCSS style variables (better SSOT) though to improve the style consistency.

Screenshots

Before	After	After (v2 review feedback)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #220249 (closed)