Role based personalization of theme programitically

Hi

We have a requirement to use role based customization of themes.

If user logs in and having role1 assigned, then use them1

If user logs in and having role2 assigned, then use them2

One method which I already know is creating different desktops for these themes.

And use main rule to load theme accordingly.

But how can this be achieved programitically.

In portal which par file is executed after user gives his login details in login screen. (Which I guess will have to be modified)

As in this par file I will user Iuserfactory api to get assigned roles.

Thanks