For the moment you should be able to run gsettings set com.solus-project.budgie-panel builtin-theme true to enable Budgie's built-in theme, which should allow you to see the settings and set any theme you like, as long as it's installed (and disable the built-in theme again if you wish).
If that fails, then you should be able to set a theme in the terminal with gsettings set org.gnome.desktop.interface gtk-theme desired-theme-name, replacing 'desired-theme-name' with the name of an installed GTK theme.
The Materia theme is installed as a dependency to the budgie-branding-fortitude package (if using Budgie Desktop), which means it should always be installed as long as the branding package is installed, but I guess for really old installs (that would be several years now, I think) it might not have that. I don't know any way around that, sadly.
I heard that the built-in theme for budgie in the development...
The current built-in theme is usable now, even if it isn't the best or prettiest, but yes, it's being completely re-done to modern standards for Budgie 10.7.
