Plata Noir is part of the plata-theme package, which is a runtime dependency of the budgie-desktop-branding-fortitude package. The reason you upgraded and it isn't installed on your system is because we do not force the installation of the branding package, or any defaults, on existing systems.
If you install the package, it will be an option under the Widgets section of Budgie Desktop Settings.
In terms of the broader question, there are multiple GTK themes in our repository that you can install that are likely to be compatible with Budgie. It is doubtful that any random theme from GNOME Look is going to work with Budgie, since there are special classes (for our popovers, budgie-menu, raven, etc) that need to be supported. You should be able to put those in ~/.local/share/themes IIRC.
