Client based branding of SaaS web application

Hi,

I am developing a SaaS based web application. My requirement is that all my users can have custom look-n-feel of the application.

Are there some standard tools to implement this.

I know I can write server side code or use javascript to update CSS and Images during page access. But is there anything I can use off the shelf?

If this question has already been asked, please point me to it.

Thanks.