Incorrect "file-holder" background color in Dark Mode

Summary

The background color on the .file-holder class is incorrect in Dark Mode. See attached screenshot.

Steps to reproduce

• Enable "Dark Mode (alpha)" as your color theme
• Navigate to the "Changes" section of a merge request
• Observe the incorrect background color displayed between changed files.

Example Project

This is reproducible on any merge request. Here is a recent random sample merge request: !142846 (diffs)

What is the current bug behavior?

The files in a merge visually request blend together due to incorrect background color

What is the expected correct behavior?

The .file-holder class should not define the background property in the way it currently does. Likely, no background color should be defined.

Relevant logs and/or screenshots

See above.

Output of checks