I have assumed that this is a plug-in issue.

I just had an idea for user-specific template sets in dynamic mode.
I know we can do it for all users by adding an argument to page.cgi (eg: /page.cgi?d=1&t=yahoo), but is it possible to set it in a cookie, so that only logged in users may switch templates (or have it set by admin)?

Hello

Suggestion you can do is create a Column in your user table called template

and then in your templates after login succesful links to
<%if Username%>
page.cgi?t=<%template%>
<%endif%>

I think that this is the easiest way to get something you want.