In other words, to override templates/saml2-post-binding.vm, you can create a file named views/templates/saml2-post-binding.vm
(path relative to idp.home).
You can find all the templates inside the opensaml-saml-impl.jar library if you want to examine or copy them:
The two additions you can make to the default Velocity templates without replacing them are to add HTML markup (actually, Velocity Template Language, which can be a mix of standard HTML markup and Velocity statements) to the
<head> section, and/or to add HTML(VTL) markup to the
<body> section. You do that by creating one or both of the following files (again relative to idp.home):