When it comes to deleting your custom cover styles, you need to consider if the cover style is in use first. If the cover style is being used anywhere, you need to make sure it's replaced!
To replace a cover style that's being used somewhere, you need to click the Merge button. You'll replace your current cover style with one from the cover style list. This will remove the custom cover style from your account and replace it everywhere with the new cover style you selected.
If the cover style isn't in use, you can simply click the delete button.