Pelican themes seem to have changed since the last time I generated.
Now there is no default theme, or at least the default theme has no css (only templates). You need to clone a github repo full of themes, and install them into /usr/lib/python3/dist-packages/pelican (!), in order to be able to reference them from the pelican config files with the THEME setting.
To clone the github repo:
cd ~/projects/pelican-stuff
git clone https://github.com/getpelican/pelican-themes.git
Install a theme into the system directory:
pelican-themes -s ~/projects/pelican-stuff/bootstrap2-dark
(or ln -s ~/projects/pelican-stuff/pelican-themes/bootstrap2-dark /usr/lib/python3/dist-packages/pelican/themes)
(or copy all the files)
Then refer to bootstrap2-dark in the pelican config files.
THEME = "bootstrap2-dark"
As far as I can tell, when pelican generates next time, it creates a themes directory in the output directory (if it doesn't already exist) and copies the contents of bootstrap2-dark from the install directory to the output/themes directory.