I'm using the gnome flashback session with metacity window manager, but I can't change to any metacity theme in any of the tweaking tools, as they're not even appearing in the lists. Is there anything I can do?
You can use GSettings to change theme:
gsettings set org.gnome.metacity theme THEME_NAME
Above will work with Metacity 3.16.x and 3.18.x, but starting with 3.20 Metacity by default will use GTK+ theme. To use Metacity themes you will need to change two settings:
gsettings set org.gnome.metacity.theme type metacity
gsettings set org.gnome.metacity.theme name THEME_NAME
Available types are
gtk. When using
gtk type Metacity will ignore THEME_NAME and will use GTK+ theme.