Added $user->theme and $course->theme choices, controlled by admin
variables on the main config variables page.
By default this is all turned off, since multiple themes can be
a little disorienting.
A hidden feature is session themes ... you can change the theme
on any page by passing theme=xxxx as a parameter. By default this
requires sesskey (so that people don't change theme on each other),
but this security can be switched off using a hidden variable
$CFG->allowthemechangeonurl = true; Good for quick tests.
