The documentation of the core components of Theano is still a work in
progress. For now this is a list of bits and pieces on the subject,
some of them might be outdated though:
Making the double type – Tutorial for writing a new type in Theano. It
introduces the basics concerning Theano datatypes.
Views and inplace operations – This is somewhere between extending Theano and
describing how Theano works internally; it talks about views and inplace
operations.