I want to customize the Episerver form by adding my own CSS rules. Is it correct if I apply custom CSS rules to existed Episerver form class names, for example, to FormTextbox and etc.? Or it is better to add your own class names to form elements? Is there any chance, that class names will be changed in a new version of CMS?
If you want to be super safe you can override the view. The location of your custom views can be configured in the ~/modules/_protected/EPiServer.Forms/Forms.config. I would say it all depends on how many elements you want to override. A details guide can be found here