Email Diff Default Color Unreadable

Summary

The HTML diff for notification email don't apply foreground and background colors consistently.

Steps to reproduce

1. Create a merge request
2. Add a second participant
3. Have the participant leave a diff comment
4. Use an email client with a dark body background
5. Open the comment notification email

Example Project

https://gitlab.com/gitlab-org/gitlab

What is the current bug behavior?

Some diff text is unreadable. Sometimes a custom light background is shown behind default light text, and custom dark text is shown on a default dark background.

What is the expected correct behavior?

The color CSS attribute on any element that has a custom background-color should ensure contrast is preserved and vice versa.

Relevant logs and/or screenshots

Light theme:

Dark theme:

Output of checks

This bug happens on GitLab.com

Possible fixes

Explicitly define color and background-color rules together in MR diff emails.