Post sub-title

How do I add a sub-title to a theme? I’ve tried a few different themes, and none of them has a sub-title. I kind of remember that at least one of them did have it, and I vaguely remember doing something to disable it, but it’s possible I’m just confused.

I guess I can use a custom field, but is that the only/preferred way?
I don’t mind making changes to the theme.