How Do I Remove Just the Background Without the Overlay Being Removed In iMovie?

I used your method of adding beat markers manually using your #992 video on YouTube. However, now that I’ve synced all my video footage to the beat, I want to remove the black background so that I can start adding in transitions between scenes. But imovie doesn’t allow me to do this without also deleting all the overlay footage on the top bar (all the video footage). Please help as I’ve just spend at least 30 hours manually marking and placing footage in the right spot. Currently using imovie 10.1.10
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Aminul Islam