A user has alerted me to this issue. Selecting a different text size in the personal settings area does not seem to modify the display of text. I’ve tried several of the standard themes. I’ve saved changes at each modification and reloaded pages. I don’t see smaller or larger text.
I’m seeing that here on meta, which is good, it means that it should work.
I’m on a hosted instance of discourse, so I’m expecting to get out-of-the-box behaviour using standard themes and our users are griping because they can’t adjust the display of text to suit their eyes.