Page by page
Page by page
I've only found you have to manually select EACH PAGE you want to center (you can grab multiple, CMD key on my Mac) in the signature view and choose "edit" "center page".
Why you can't set as a default? Don't know... although it's ridiculous. Anyway, if you have a lot you may want to adjust your page boxes in PitStop instead, or you will get pretty good at manually setting your page geometry like we do on most multi-page documents that don't have correct page information. If you're doing a whole book, select all in the "page" section and set the geometry for all.
If anyone DOES know how to set that as a default, I'd love to set mine that way.