Activate theme via plugin script?

switch_theme() should work:

function myplugin_activate() {
  switch_theme('default', 'default');
}

register_activation_hook( __FILE__, 'myplugin_activate' );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)