Triggered when a default theme is changed by a user.
The handler method should accept two parameters - the old and new default theme objects. You can
throw an exception in the handler to stop the operation.
public function subscribeEvents()
{
Backend::$events->addEvent('cms:onSetDefaultTheme', $this, 'set_default_theme');
}
public function set_default_theme($old, $new)
{
// Do something
}