Try to fix dark theme unlinked files

[Siedlerchr]

Fixes #9481

• 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 developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[ThiloteE]

self-closed because

a) colour of symbols still differ
b) we found another regression as compared to JabRef 5.7: when there are multiple files, the background colour of linked files remains orange. Only found but unlinked files shoudl remain orange. To be honest, orange is maybe also a little aggressive of a colour.