I'm looking at installing BuddyPress but can you clarify

Hi,
I'm looking at installing BuddyPress but can you clarify about the themes please?

Am I right that I need to install a new theme on my main site? I don't want to do that as it's already set up and styled or is that something I have to do if I want to use BP?

One other question - I've seen that BP only works on multisite but also that it works on a single site. Which is correct?

Thanks
Gareth