Hi Ger,
im not sure if this thread is still alive, BUT...
the session theme sets a theme that remains until the user logs out. When the users next logs in, the site/course/user themes are active again.
I'm currently managing a site with about 1300 users, 200 of which are PocketPC users. So the session theme is particularly useful for our PocketPC users who need to view Moodle at a different resolution.
What to do:
1. Go to config.php located in moodle base directory, add URL parameter:
$CFG->allowthemechangeonurl = true;
now lets assume your parameter is &theme=orangewhitepda
2. You can provide a URL [containing a course wildcard '*'] for them on the main page of the site eg.
http://mymoodlesite.org/course/view.php?id=*&theme=orangewhitepda
3. You may also provide a specific URL to a course eg.
http://mymoodlesite.org/course/view.php?id=25&theme=orangewhitepda
this will take the user directly to course id = 25 using orangewhitepda theme
i hope this helps somebody! cheers, Chad