if you open settings (from O menu) you can set default zoom for all pages like this:
http://prntscr.com/caaf04
also, if you change zoom on any page, whole domain will use that zoom (eg. if you change zoom on this page, all forum pages will have that zoom and it will be saved for any later visit until you change it again). You can do that also from O menu (arrows left and right)
there is an third option to zoom pages only currently (not saved) and only active page (not the whole domain), but that is available only trough extensions