After I change "SSViewer::set_theme('tutorial');" to "SSViewer::set_theme('myfirsttheme');" in the mysite/_config.php file, I get an error saying "Please check whether if this file is opened in another program" when I try to save it. But I dont have this file open in anywhere else.
I've got the problem solved. The reson was my Apache user account couldn't access the config file. So I copied the config file to my desktop and made the change then pasted the changed one back to the mysite folder. It works after that.