On this website, which was dealing with e.g.
xubuntu not debian the commands recommended to
install it were:
sudo add-apt-repository ppa:rebuntu16/other-stuff
sudo apt-get update
sudo apt-get install xfce-theme-manager
I think the reason it may not be working in my
case is that I don't know what the correct
equivalent of
sudo add-apt-repository ppa:rebuntu16/other-stuff
would be for a debian user.