The setting $conf['theme'] = 'default'; is used for the login window when no user is logged in. The theme of a client is set in the client settings only.
So upon further googling it turns out I could edit the default module in interface\lib\config.inc.php but new problem arise. The Start module is now change to Client. When the client is first login they will see client menu inside client menu There's no option anywhere in config.inc.php to set client default Start module.
The client module is activated / deactivated automatically depending if it is a reseller or client as clients may never see the client module so it my not be specified in the default modules.