No, you can make your own, even from the board admin panel (at least for SMF). For SMF, you need to copy the default theme from there, though, and it's not very easy to edit everything.

Also SMF 2.0 being in beta doesn't mean it's unstable. It got updated quite a bit and most people hardly found any issues so far. Some people even use it fine on their site. The only issue is that after it gets updated, some old mods or themes might stop working.