When I select the Default theme, it looks like Adwaita. When I look in the /usr/share/themes directory, I see a folder for Adwaita, but no folder named Default. OK, so far so good.
However, back in Budgie Desktop Settings with the Default theme selected, when I switch on Dark theme nothing happens. There is an Adwaita-dark folder in /usr/share/themes but it apparently Budgie Desktop Settings does not use it. However I found a workaround.
If I copy the Adwaita-dark folder and paste it into /.themes there is no change. I stall can’t see the theme in Budgie Settings. However, if I change the name of the /.themes/Adwaita-dark to Nodwaita-dark, I can see the Nodwaita-dark theme in Budgie settings, and it works properly, or at least how I expect Adwaita Dark to look like.
So that’s one anomaly, or bug if you want to call it that. Adwaita Dark theme is installed by default (I think) but not usable in a default install.
The second anomaly happens when I download a theme from gnome-look.org with Adwaita in the name. It could be Adwaita-Green, Adwaita-dark-blue, Adwaita-whatever. These themes are not recognized by Budgie Desktop Settings. I can make the themes show up in Budgie Desktop Settings by changing the Adwaita part of the name to Nodwaita.
I am using an Adwaita based theme that I like, and I can try out any Adwaita based theme with just a little extra work. So I’m not really complaining here, just pointing out a workaround for something that is unusual.