If you set your Homepage to something other than None or Categories, there is no way to get to Categories (that is, the list of sub-forums) aside from modifying the URL, because Categories is missing from the left side menu you get by clicking the three lines icon.

My guess is that this is an oversight.

The “hamburger menu” should be gone now that I’ve changed the underlying theme. And i found the way to add back in the three-bullet-points icon between “community” and “inbox” that takes you to “Category”, so it should be closer to what you expect.