I'm hoping someone out there can help me out with how I can set the theme for a site based on the user. Here's what I'm trying to accomplish:

When someone initially comes to the site, the default theme is displayed. Once the user logs in, I'd like to have the theme change to whichever theme has been associated with that specific user.

The reason I'd like to work out this functionality is that some of my site visitors are competitors to one another and they are very sensitive to the color schemes they see.

Does anyone out there have the knowledge of how to do this?