Unfortunately I’m not going to be of much help. If you’ve changed themes and you are still seeing ACF fields (do you mean options pages), then whatever is being done could be done in a plugin.
You will also need to trash/delete the field groups that have been created. These are stored in the DB and will be there even after changing themes.
I can’t give much more advice on what would need to be done, it’s going to be specific to whatever the other dev did in the first place.