The bug
Much like how the black "currently selected" box used for resource packs and data packs cannot be retextured (MC-269443), the red rectangle indicating an incompatible pack cannot be retextured either, and appears to be hardcoded,
How to fix
Introduce a texture file for this effect (an opaque red rectangle with appropriate scaling mcmeta should suffice).
Linked issues
relates to 1
Attachments
Comments 2
Affects 1.20.5 pre-release 3.
As far as I can make out this is not present in the assets folder of the game .jar