This more or less completes the PHP side of the 1.5 themes

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.

lib/defaults.php

@@ -5,6 +5,8 @@
 
    $defaults = array (
        'allowunenroll'            =>  true,
+       'allowcoursethemes'        =>  false,
+       'allowuserthemes'          =>  false,
        'allusersaresitestudents'  =>  true,
        'auth'                     => 'email',
        'auth_pop3mailbox'         => 'INBOX',
@@ -66,6 +68,7 @@
        'teacherassignteachers'    =>  true,
        'template'                 => 'default',
        'textfilters'              => 'mod/glossary/dynalink.php',
+       'themelist'                => '',
        'timezone'                 =>  99,
        'theme'                    => 'standard',
        'unzip'                    => '',