Action Form produces an XML file which is used by the XSL file to define the form structure and to render the form fields - the bootstrap template it’s based on this XSL file. This XML template could be visible just by putting Action Form in Debug Mode.
Following this path: \DesktopModules\AvatarSoft\ActionForm\templates\Form, you will find the bootstrap template. You can edit this default template or just make a new one with copy/paste and renaming it so you can differentiate them. After you make these changes, your custom template will appear in the drop-down selection.