No, it’s not theft if it is offered to you. But if you are using the WiFi of another party without their knowledge and permission, it is prosecutable. The party who is unwittingly providing you with access may not be amenable to the situation.
In my business, I do a lot of WiFi setups. I always recommend that they use a password-protected system. You never know who might be driving by, looking for unprotected WiFi systems.
I live on a corner in my neighborhood. One of my neighbors wi-fi is unsecured. I've told them about it, they didn't seem to care. One of these days they're going to get hacked and their identity stolen by one of the kids around here who are always looking for open networks. It's pretty easy to tell who those kids are, they typically have a small yagi somewhere clipped to their gutter or outside an upstairs window pointing somewhere.
I live on a corner in my neighborhood. One of my neighbors wi-fi is unsecured. I've told them about it, they didn't seem to care. One of these days they're going to get hacked and their identity stolen by one of the kids around here who are always looking for open networks. It's pretty easy to tell who those kids are, they typically have a small yagi somewhere clipped to their gutter or outside an upstairs window pointing somewhere.