Custom icons are removed in much the same way as they are added. First, add folders to the folder list, if you haven't already To add folders, drag them from the Finder to the list, use the button below the list, or use the "Add Folders to List..." entry in the "File" menu.
Adding Folders to the List
If you add a folder by mistake, select it then click on the button to remove it from the list.
Finally, click on the "Remove Folders" button. Add Folder Icons will check to make sure you really want to proceed:
Making sure you want to proceed
...then it will take the custom icons away. Standard special icons for things like your Documents, Downloads or Photos folders in your user account will be restored correctly. Other "ordinary" folders will be restored to the standard folder icon.
Icon removal is underway