How do I enable the Dark Mode in Windows Beta?

Just downloaded the Windows Beta version - it is supposed to have pre-set colour modes, including two dark modes.
Problem is; it’s not clear how to enable them. I think I looked in all the usual places. Any beta user out there able to tell me?

The Window menu, Themes. Should be listed.

As a note for future reference, any beta-related questions should go in the beta forum.