remove red/green colors from diff view of no-color syntax theme

What does this MR do?

When we added the no-color syntax highlighting theme (!20170 (merged)) I was asked to leave the 'diff view' parts of the CSS for a separate merge request.

The current red/green output for the diff request is difficult to read for people with some kinds of color blindness. This merge request desaturates the colors, leaving them shaded (to indicate they are changed) and strengthens the contrast of the +/- sigils (to make it easier to tell which lines are which). In addition, instead of indicating intra-line changes with color saturation, it decorates the diff spans with underlines, for easier identification of these changes without relying on color vision.

What are the relevant issue numbers?

#32109 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• 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 e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer