I cannot delete a Display Rules Set, Why the delete Display Rules Set icon is greyed out?
The reason for this is that the Display Rules used are being defined in a dgnlib, so the Display Rules exist only in the DGNlibrary. They are not saved locally in the file.
If you move the mouse on top of each Display Rules Set you can see this information
See Screen shots:
If we change the conditions or actions in these Display Rules Set that exist only in the DGNlibrary the changes will be saved locally on the dgn in use.
So if you need to change conditions or actions you can do it so then they are locally saved otherwise you can move the DGNlibrary to a different folder where it will not be read or just delete the DGNlibrary in use or modify the DGNlibrary with new Display Rules Set.