Am assuming you mean the ' Subscribe' button that appears at the bottom of a topic (posts) view.
Just tested with CSS-Only (but V2) and I still see the button on my test system. It IS in a different location - over on the left instead of the right - but it still shows up.
Please confirm this is the button you are referring to.
I am in the process of rebuilding my SP V5.7.x development system but I hope to have that done later today and will re-test at that time.
yes, that's what i meant, and yes, the subscribe button appear in both theme
but i realized theme was not the problem, when i saw my site as an admin, i can saw the subscribe button, but when i saw my site as a member, the subscribe button disappear.
is there any setting that i should do or shouldn't do in the subscribe setting?
or does this problem related with permission setting?
This will be down to permissions- yes.
Check which permission sets you are using for your forums and then go to the permissions management admin section and edit the sets you need to allow the right users to subscribe - and then turn in the required actions.
For added convenience - there is a button on the permission admin page listing (permission usage) that shows which forums and user groups are set to use it.