Hi Pradip,
Putting themes in the foundation layer, has nothing to do with CSRF attackes. Please read here how to Protecting OutSystems apps from Cross Site Request Forgery attacks.
The Architecture Canvas is an OutSystems architecture tool to make the design of Service-Oriented Architectures (SOA) simple. It promotes the correct abstraction of reusable (micro)services and the correct isolation of distinct functional modules, in cases where you are developing and maintaining multiple applications that reuse common modules.
Putting a theme in the foundation as such does not expose it to external connections, but makes it reusable for multiple OutSystems applications.
Regards,
Daniel