Fix diff files sticky header transparent background

What does this MR do and why?

Relates to !76970 (merged)

Right now commit page file header does not have a background when it's stuck, this MR intends to fix this.

Example page: 9e89cea3

Screenshots or screen recordings

Before	After	After (Dark Mode)

How to set up and validate locally

1. Open any commit page with enough lines to full up the screen
2. Scroll down to see sticky commit header

MR acceptance checklist

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