Should a web designer know server-side coding?

Question

We're implementing an CMS based on ASP.NET MVC. Now, any designer should be able to provide themes for this CMS. But to write a theme, they need to be able to modify the generated HTML, thus the concept of View. In other words, they should be capable to either modify current views, or create new views from scratch, to fit their requirements. However, now we're not sure that we're taking the right path.

Should a web designer (HTML, CSS, JavaScript + Photoshop) really know about server-side platforms like Razor or PHP, or classic ASP, or anything else?