Torus and Inter are what I got from Inspecting (F12 in browser) the site.
And depending on the language you're currently setting, the site may give you a different font in order to render characters in that languages.
E.g. for Vietnamese, I got Quicksand.
For CJK languages, it mostly depends on your Operating System.
E.g. in Windows, you'd get Microsoft YaHei & JhengHei for Chinese (Simplified & Traditional), and Meiryo for Japanese iirc.