change color in darktheme

[mohanNaryana]

this fixes the problem #9192

• 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.

[HoussemNasri]

Please provide a screenshot to showcase the new UI changes. Also, the changes you made don't seem to edit the syntax highlighting colors for the three-way-merge UI.

[Siedlerchr]

Closing this issue due to inactivity 💤
Please reopen the issue with additional information if the problem persists.