I may be wrong but what I understood is that currently you have wordpress-like setup for theming your web application : Theme developers upload theme files(html/css/images) to publish it on production server. What you now need is the ability of average non-technical user to modify and save any standard theme online without the need to touch html/css code. Something similar to tumblr : http://oi39.tinypic.com/339lpfk.jpg

Right? I don't know about any SAAS that will let you do that and even if it does it will be difficult to integrate with your system. Why not develop it yourself or outsource it to some professional developer? All you need to do is-

- present copy of a default theme to customize

- let user change text using inline editing

- let user change/upload images

- let user select different colors/fonts using forms

- optional html/css editor for advanced users

- Save and apply the changes