Programming exercises: Fix a visual issue with static code analysis warnings in the online editor

[b-fein]

Checklist

General

• I tested all changes and their related features with all corresponding user types on a test server.
• This is a small issue that I tested locally and was confirmed by another developer on a test server.
• Language: I followed the guidelines for inclusive, diversity-sensitive, and appreciative language.
• I chose a title conforming to the naming conventions for pull requests.

Client

• I followed the coding and design guidelines and ensured that the layout is responsive.
• Following the theming guidelines, I specified colors only in the theming variable files and checked that the changes look consistent in both the light and the dark theme.
• I added multiple screenshots/screencasts of my UI changes.

Motivation and Context

Closes #6366.

Description

Sets the colour of the diff as background-colour instead of background. Therefore, the image for the warning is not replaced, but instead remains with the colour now in the background behind it.

Steps for Testing

Prerequisites:

• 1 Instructor
• 1 Student
• 1 Java Programming Exercise with manual assessment

1. Submit as student to the exercise a submission that contains static code analysis issues. E.g. introduce an attribute public static int non_uppercase = 0;
2. Start the manual assessment as instructor.
3. The issue should be shown in the gutter like in the screenshot below in the online code editor view.

Review Progress

Code Review

• Review 1
• Review 2

Manual Tests

• Test 1
• Test 2

Screenshots

For screenshots from before, check #6366.

Warning is shown with green background:

[Strohgelaender]

code