bbpress doesn’t have theme’s – it uses your sites theme, but sometimes integration is not great.
Unfortunately your link produces a certificate error (The security certificate presented by this website was issued for a different website’s address.) on the https side, so I didn’t visit.
If it is just styling rather than function try my plugin
https://wordpress.org/plugins/bbp-style-pack/
Otherwise give us a valid link, and I’ll take a quick look