How to have skin selection in the hamburger dropdown like THIS forum?

I have the official dark theme here, but I had to manually add it since I had Discourse for a couple years now (following the official instructions). I’m going to safely assume that those instructions did not include how to add skin selection to this dropdown instead of only in your user profile.

How can I add this?

It’s actually a theme that you can add as a component to other themes. Details here: