Add transparent background to markdown header tabs

What does this MR do?

Markdown header tabs were changed to buttons in !21988 (merged). This caused a minor style inconsistency on some user agents.

Before	After

What are the relevant issue numbers?

Fix #53105 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has ~"Requires e2 tests" label. See the Test Planning Process.