Fixed incorrect color of identity column header

[Gena928]

Fixes #6796
Small adjustment - added style for identity column header.

• Change in CHANGELOG.md described (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.

See column with "#" in header:

[IsaacRoles]

#6796 (comment)

This may be another possible solution to the issue.

[Siedlerchr]

As outlined in the issue, just introduce a new css class for that Node. You just have to define that css class then in then in the Base.css and the Dark.css

[Gena928]

@Siedlerchr
OK. I am a bit busy now, so I'll look into this tomorrow.