Fix: re-implemented bib entry highlight when dragging and dropping files

[Nicholas-An]

Closes #11898
Removed a chunk of code that was overriding the background colour of bib entires when dragging and dropping a file. Initially caused by changes implemented when resolving #11579.

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• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
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[Siedlerchr]

I am not sure that removing all the colors is really a good idea, they were implemented for the different searching options ("floating mode") with groups and without groups

[calixtus]

Yes, this would remove the visual distinction between matching groups in floating search mode.
Please reimplement the matching/non-matching colors. Highlighting must respect them. Maybe @loay can help here?

[Nicholas-An]

Ah that makes sense! I thought for some reason the deleted section was redundant, I'll rework it to work around the matching groups in floating search mode. Thanks for the feedback!