hello peiyouliao,
not entirely sure if I understand you correctly but are you saying that you want to switch away from the optimum theme to another one but you see no difference in the frontend when doing so?
if so, the reason why changing the theme has no visual effect for you right now is because in your profile type settings you selected optimum as theme as well which will override your global theme selection, see here:
http://screencast.com/t/G5R1SErR
I changed this now to use the "default theme" (which is still optimum as of your default theme settings) but from now on when you select another template you should see the difference.
hope that helps but if you face any further issues on this feel always free to get back to us at any time. have a nice day