Change theme

The changetheme module creates a block that can be activated in any region
of all the themes that will be used.

• For each group, the administrator can select which themes are going to be
  enabled for the user.
• The block show the user a list of all enabled themes and allow them to
  choose between them.
• The user may change the theme in each group.
• Inside of a group, the theme can be changed at any moment.
• For logged in users, the user options is updated so that their last
  selection will stay the same the next time they log in.
• Anonymous users have their choices saved in the session variable.