Fix for issue 5522 and some UI changes.

[mohit038-zz]

Fixes #5522

1. Adding patterns to the entries background in dark theme.

2. Visibility issue with the group item number coloration when an item is assigned to a new group as mentioned in Dark theme: Visibility problems with current colouring scheme (group colouring and scroll bars) #5522 (comment)

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[tobiasdiez]

I think the insets and padding don't need to be set here again, as they are controlled by the light theme, right?

[mohit038-zz]

Yes, you are right. I'll change it.

[tobiasdiez]

Thanks!

[Siedlerchr]

Thanks a lot for your contribution!