When a sheet was protect, It's impossible to modify or delete a style. If you are 20 sheets, you have to deprotect all sheets before modify a style and reprotect all sheets. Further no alert/popup for inform the user of this problem. When you click on modify or delete, nothing appear. Thanks
Thanks Madvic, I clarify a bit and set to new
*** This bug has been marked as a duplicate of bug 57102 ***