You're design team should not need to have any knowledge of the view engine at all. They should only need to know about the final product from the view engine (i.e. the HTML, CSS and Javascript that is output).
Your designers can make templates from plain old HTML and CSS, without ever seeing a single line of rendering engine code. You just have to tell them the places in the template you are going to inject the content.
The whole point of CSS/HTML templates is to provide separation between the designer and the developer. This allows these templates to be farmed-out to a design shop. You don't want the design shop to have to mess with your development code.
The designer will also be providing you with a set of text styles: h1, h2, h3, p ,etc. You will be able to plug those styles in wherever you need them in the templating code of the rendering engine to achieve the desired effects. If you wish, you can let the designer dictate some rules about the layout and use of these styles, but it's still your job to write the code that renders the output into the designer template.
So to be clear, the designer's job is to create an HTML/CSS template for you (with sample content and styling so that both of you can adequately see the layout). Your job is to incorporate the CSS/HTML the designer provides you into the view engine code.
Spark is just an HTML-ified version of C# (or VB). All other things being equal, Spark would be easier for a designer, because it changes all of the <% { %>
things to HTML equivalents. But that assumes that the designers will be writing the template code for the view engine, which they won't be.