Yes, that is the folder I was pointing to. The original themes are also stored in the installation folder of CP. This is a copied folder. If you delete it, a new copy with only the default themes will be made after reopening Captivate.
Captivate 8.0.1 Install? Keep your Customisation! - Captivate blog
But if you have a lot of custom themes, those will be deleted. This is the last possible work flow if you cannot get out of the problems with your custom buttons in conflict with custom themes.