You can allow them to modify things like themes under your own control. But what about actual content? What CONTENT can you allow them to manipulate? This is a big risk in terms of security and privacy over all. Most web services because of this limit it to highly predefined things like some text.
So how am I looking to do it? First off there is two portions to a system that I am suggesting.
- Front end, where the actual editing goes on.
- Back end, verification and manipulation of site content.