Css/Dtd

I have been skimming through the Openmrs source code. I am wondering if there is a document that explains the various components of Openmrs and how they interact, like a schematic diagram

In using JSP, is there a predefined CSS and/or Dtd files that are being used and where can one find their definations?
In my conversation with Ben, I have the impression that there are a number of methods that are already implemented in openmrs that one would like to borrow from but the structure of openmrs seems deep to understand in a short time and develop at the same time. for developers who are used to almost procedural top-bottom approach, developing for Openmrs would be slowed down majorly by the mind set. Even if one is good in Java, these skills may prove difficult to use.