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.