The closest solution i can came up right now is this.
if (Configure::read('Site.admin_theme') != '') {
                $this->theme = Configure::read('Site.admin_theme');
}
I'm not sure if that's the right solution. I try to find $adminTheme variable on the whole application to see if its define on other code stuff but i can't find one. Just let me know if its ok with you so I can patch up the code.