Simon: It's more than one line - what if someone creates a theme and forgets the 'if' for instance?
Then properly someone would find out and the iff-statement could be added. Unless the feature was implemented such that the defaulted to e.g. a pirate flag, in which case I would suppose it would be the same in all themes.
Should the option be opt-in or opt-out?
I dont really care. Is this not simply a matter of "alter table users add column show_flag bool default='false'"?
Does it apply to new accounts and old in the same way?
Why not just the same?