I have been trying to customize the icons in my DropBox repository.
This works partly, but erratically and mostly is overwritten by DropBox default icons.
I would suggest that you set the desired icons locally in Dropbox and then save them as a setting online, so they will be reapplied when Dropbox synchronizes from the cloud.
Apparently restoring the Dropbox icons is not consequent, as sometimes a custom icon survives through synchronizing...
In the image, the Nedcom Logo survived between Synchronizations, but last time, it too was restored to a folder icon, while then the Camera-uploads icon was left intact.