Skinning scalability

[John Smith]

We are trying to achieve a flexible UI which will be driven off of Themes
and Skins. Since we are going to market the application to different
clients - we want to be able to change the look and feel of the application
quickly.

We are thinking of using Skins for everything (images, colors, fonts etc.)
The question I am curious about is how scalable is this solution than using
plain old HTML.

Thanks
John