I assume this has to do with the shift to the HiDPI version – custom icons imported or brought into Scrivener now appear blurry and pixelated. I’m assuming this is a bug, because the problem can be fixed from outside the Scrivener program, by copying the custom icons directly into the ‘icons’ folder, where reuploading them into the project itself results in the same pixelation.
To my admittedly untrained eye, it seems that the Scrivener program automatically resizes imported custom icons to a smaller resolution not intended for HiDPI, while icons copied directly into the folder bypass this resizing and thus appear at their proper, unpixelated resolution.